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verb (used with object)
  1. to make flat.
  2. to knock down: The boxer flattened his opponent in the second round.
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verb (used without object)
  1. to become flat.
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Verb Phrases
  1. flatten in, Nautical. flat1(def 61).
  2. flatten out, Aeronautics. to fly into a horizontal position, as after a dive.
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Origin of flatten

First recorded in 1620–30; flat1 + -en1
Related formsflat·ten·er, nouno·ver·flat·ten, verb (used with object)un·flat·tened, adjective


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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for flatten


  1. (sometimes foll by out) to make or become flat or flatter
  2. (tr) informal
    1. to knock down or injure; prostrate
    2. to crush or subduefailure will flatten his self-esteem
  3. (tr) music to lower the pitch of (a note) by one chromatic semitoneUsual US word: flat
  4. (intr foll by out) to manoeuvre an aircraft into horizontal flight, esp after a dive
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Derived Formsflattener, noun
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 flatten


late 14c., "to prostrate oneself," also "to fall flat," from flat (adj.) + -en (1). Meaning "to make flat" is 1620s. Related: Flattened; flattening.

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Online Etymology Dictionary, © 2010 Douglas Harper