adjective, deep·er, deep·est.
  1. extending far down from the top or surface: a deep well; a deep valley.
  2. extending far in or back from the front or from an edge, surface, opening, etc., considered as the front: a deep shelf.
  3. extending far in width; broad: deep lace; a deep border.
  4. ranging far from the earth and sun: a deep space probe.
  5. having a specified dimension in depth: a tank 8 feet deep.
  6. covered or immersed to a specified depth (often used in combination): standing knee-deep in water.
  7. having a specified width or number of items from front to back (often used in combination): shelves that are 10 inches deep; cars lined up at the entrance gates three-deep.
  8. extending or cutting far down relative to the surface of a given object: The knife made a deep scar in the table.
  9. situated far down, in, or back: deep below the surface; deep in the woods.
  10. reaching or advancing far down: a deep dive.
  11. coming from far down: a deep breath.
  12. made with the body bent or lowered to a considerable degree: a deep bow.
  13. immersed or submerged in or heavily covered with (followed by in): a road deep in mud.
  14. difficult to penetrate or understand; abstruse: a deep allegory.
  15. not superficial; profound: deep thoughts.
  16. grave or serious: deep disgrace.
  17. heartfelt; sincere: deep affections.
  18. absorbing; engrossing: deep study.
  19. great in measure; intense; extreme: deep sorrow.
  20. sound and heavy; profound: deep sleep.
  21. (of colors) dark and vivid: a deep red.
  22. low in pitch, as sound, a voice, or the like: deep, sonorous tones.
  23. having penetrating intellectual powers: a deep scholar.
  24. profoundly cunning or artful: a deep and crafty scheme.
  25. mysterious; obscure: deep, dark secrets.
  26. immersed or involved; enveloped: a man deep in debt.
  27. absorbed; engrossed: deep in thought.
  28. Baseball. relatively far from home plate: He hit the ball into deep center field.
  29. Linguistics. belonging to an early stage in the transformational derivation of a sentence; belonging to the deep structure.
  1. the deep part of a body of water, especially an area of the ocean floor having a depth greater than 18,000 feet (5400 meters).
  2. a vast extent, as of space or time.
  3. the part of greatest intensity, as of winter.
  4. Nautical. any of the unmarked levels, one fathom apart, on a deep-sea lead line.Compare mark1(def 20).
  5. the deep, Chiefly Literary. the sea or ocean: He was laid to rest in the deep.
adverb, deep·er, deep·est.
  1. to or at a considerable or specified depth: The boat rode deep in the water.
  2. far on in time: He claimed he could see deep into the future.
  3. profoundly; intensely.
  4. Baseball. at or to a deep place or position: The outfielders played deep, knowing the batter's reputation as a slugger.
  1. go off the deep end,
    1. to enter upon a course of action with heedless or irresponsible indifference to consequences.
    2. to become emotionally overwrought.
  2. in deep,
    1. inextricably involved.
    2. having made or committed oneself to make a large financial investment.
  3. in deep water,
    1. in difficult or serious circumstances; in trouble.
    2. in a situation beyond the range of one's capability or skill: You're a good student, but you'll be in deep water in medical school.

Origin of deep

before 900; Middle English dep, Old English dēop; akin to Gothic diups, Old Norse djupr, Old High German tiof
Related formsdeep·ness, nounnon·deep, adjectiveo·ver·deep, adjectiveun·deep, adjectiveun·deep·ly, adverb

Synonyms for deep

Antonyms for deep

1, 10, 15–17, 23. shallow. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for in deep


  1. extending or situated relatively far down from a surfacea deep pool
  2. extending or situated relatively far inwards, backwards, or sidewaysa deep border of trees
  3. cricket relatively far from the pitchthe deep field; deep third man
    1. (postpositive)of a specified dimension downwards, inwards, or backwardssix feet deep
    2. (in combination)a six-foot-deep trench
  4. coming from or penetrating to a great deptha deep breath
  5. difficult to understand or penetrate; abstruse
  6. learned or intellectually demandinga deep discussion
  7. of great intensity; extremedeep happiness; deep trouble
  8. (postpositive foll by in) absorbed or enveloped (by); engrossed or immersed (in)deep in study; deep in debt
  9. very cunning or crafty; deviousa deep plot
  10. mysterious or obscurea deep secret
  11. (of a colour) having an intense or dark hue
  12. low in pitch or tonea deep voice
  13. go off the deep end informal
    1. to lose one's temper; react angrily
    2. mainly USto act rashly
  14. in deep water in a tricky position or in trouble
  15. throw someone in at the deep end See end 1 (def. 28)
  1. any deep place on land or under water, esp below 6000 metres (3000 fathoms)
  2. the deep
    1. a poetic term for the ocean
    2. cricketthe area of the field relatively far from the pitch
  3. the most profound, intense, or central partthe deep of winter
  4. a vast extent, as of space or time
  5. nautical one of the intervals on a sounding lead, one fathom apart
  1. far on in time; latethey worked deep into the night
  2. profoundly or intensely
  3. deep down informal in reality, esp as opposed to appearanceshe is a very kind person deep down
  4. deep in the past long ago
Derived Formsdeeply, adverbdeepness, noun

Word Origin for deep

Old English dēop; related to Old High German tiof deep, Old Norse djupr
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for in deep



Old English deop "deep water," especially the sea, from the source of deep (adj.).



Old English deop "profound, awful, mysterious; serious, solemn; deepness, depth," deope (adv.), from Proto-Germanic *deupaz (cf. Old Saxon diop, Old Frisian diap, Dutch diep, Old High German tiof, German tief, Old Norse djupr, Danish dyb, Swedish djup, Gothic diups "deep"), from PIE *dheub- "deep, hollow" (cf. Lithuanian dubus "deep, hollow, Old Church Slavonic duno "bottom, foundation," Welsh dwfn "deep," Old Irish domun "world," via sense development from "bottom" to "foundation" to "earth" to "world").

Figurative senses were in Old English; extended 16c. to color, sound. Deep pocket "wealth" is from 1951. To go off the deep end "lose control of oneself" is slang first recorded 1921, probably in reference to the deep end of a swimming pool, where a person on the surface can no longer touch bottom. When 3-D films seemed destined to be the next wave and the biggest thing to hit cinema since talkies, they were known as deepies (1953).

Online Etymology Dictionary, © 2010 Douglas Harper

Idioms and Phrases with in deep

in deep


Seriously involved; far advanced. For example, He was in deep with the other merchants and couldn't strike out on his own, or She used her credit cards for everything, and before long she was in deep.


in deep water. Also, in over one's head. In trouble, with more difficulties than one can manage, as in The business was in deep water after the president resigned, or I'm afraid Bill got in over his head. These metaphoric expressions transfer the difficulties of being submerged to other problems. The first appears in Miles Coverdale's 1535 translation of the Book of Psalms (68:13): “I am come into deep waters.” The second, which also can signify being involved with more than one can understand, dates from the 1600s. Also see over one's head.


In addition to the idioms beginning with deep

  • deep down
  • deep end
  • deep pocket
  • deep six
  • deep water

also see:

  • beauty is only skin deep
  • between a rock and a hard place (devil and deep blue sea)
  • go off the deep end
  • in deep
  • still waters run deep
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.