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• must
  must

  auxiliary verb

  to be obliged or bound to by an imperative requirement.
• must-
  must-

  combining form

  indicating that something is highly recommended or desirable

must

¹ American  

[muhst] / mʌst /

auxiliary verb

1. to be obliged or bound to by an imperative requirement.

   I must keep my word.

2. to be under the necessity to; need to.

   Animals must eat to live.

3. to be required or compelled to, as by the use or threat of force.

   You must obey the law.

4. to be compelled to in order to fulfill some need or achieve an aim.

   We must hurry if we're to arrive on time.

5. to be forced to, as by convention or the requirements of honesty.

   I must say, that is a lovely hat.

6. to be or feel urged to; ought to.

   I must buy that book.

7. to be reasonably expected to; is bound to.

   It must have stopped raining by now. She must be at least 60.

8. to be inevitably certain to; be compelled by nature.

   Everyone must die.

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verb (used without object)

1. to be obliged; be compelled.

   Do I have to go? I must, I suppose.

2. Archaic. (sometimes used with ellipsis of go, get, or some similar verb readily understood from the context).

   We must away.

adjective

1. necessary; vital.

   A raincoat is must clothing in this area.

noun

1. something necessary, vital, or required.

   This law is a must.

must ² American  

[muhst] / mʌst /

noun

1. new wine; the unfermented juice as pressed from the grape or other fruit.

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must ³ American  

[muhst] / mʌst /

noun

1. mold; moldiness; mustiness.

   a castle harboring the must of centuries.

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must ⁴ American  

[muhst] / mʌst /

noun

1. musk, especially a powder made from musk.

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verb (used with object)

1. to powder (the hair).

must ¹ British  

/ məst, mʌst, məs /

verb

1. used as an auxiliary to express obligation or compulsion: you must pay your dues . In this sense, must does not form a negative. If used with a negative infinitive it indicates obligatory prohibition

2. used as an auxiliary to indicate necessity

   I must go to the bank tomorrow

3. used as an auxiliary to indicate the probable correctness of a statement

   he must be there by now

4. used as an auxiliary to indicate inevitability

   all good things must come to an end

5. 1. on the part of the speaker when used with I or we

      I must finish this

   2. on the part of another or others as imputed to them by the speaker, when used with you, he, she, they, etc

      let him get drunk if he must

6. (used emphatically) used as an auxiliary to express conviction or certainty on the part of the speaker

   he must have reached the town by now, surely

   you must be joking

7. (foll by away) used with an implied verb of motion to express compelling haste

   I must away

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noun

1. an essential or necessary thing

   strong shoes are a must for hill walking

must ² British  

/ mʌst /

noun

1. mustiness or mould

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must ³ British  

/ mʌst /

noun

1. the newly pressed juice of grapes or other fruit ready for fermentation

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must ⁴ British  

/ mʌst /

noun

1. a variant spelling of musth

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must- ⁵ British  

combining form

1. indicating that something is highly recommended or desirable

   a must-see film

   this season's must-haves

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must Idioms  

1. see a must; show must go on.

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Synonym Usage

Must, ought, should express necessity or duty. Must expresses necessity or compulsion: I must attend to those patients first. Soldiers must obey orders. Ought (weaker than must ) expresses obligation, duty, desirability: You ought to tell your mother. Should expresses obligation, expectation, or probability: You are not behaving as you should. Children should be taught to speak the truth. They should arrive at one o'clock.

Etymology

Origin of must¹

First recorded before 900; Middle English most(e), Old English mōste (past tense); cognate with German musste. See mote ²

Origin of must²

First recorded before 900; Middle English, Old English, from Latin mustum, short for vīnum mustum “new wine”

Origin of must³

First recorded in 1595–1605; back formation from musty ¹

Origin of must⁴

First recorded in 1480–90; earlier moist, from Middle French must, variant of musc; see origin at musk

Explanation

A must is something that's so important it can't be missed or ignored. When you're taking kayaking lessons, a lifejacket is a must. Anything that is required is a must: drinking enough water is a must for being healthy. When you adopt a new dog, making sure he'll get along with your cat is a must. Another meaning of the noun must is a stale, stuffy smell. Your damp basement and dry, hot attic might both smell of must. This meaning comes from the adjective musty, a variation on the older, now obsolete moisty.

Example Sentences

Its single “Heaven Must Be Missing an Angel” reached No. 15 on the Billboard Hot 100 and sold a half-million copies by September.

From The Wall Street Journal • May 25, 2026

"We were 40-50 runs short. Must commend the guys in the bottom half for giving us a chance."

From Barron's • Feb. 26, 2026

I do a three-list rule, where I sit with a legal pad and I make three lists: Must do, should do and don’t you dare add this today.

From The Wall Street Journal • Dec. 8, 2025

BANNING: Must break in new quarterback but lots of players to build around, including linebacker Keshawn Galloway and defensive back Alonzo Ruiz.

From Los Angeles Times • Aug. 1, 2025

“Remember all the American soldiers in trucks yesterday? Must have been twenty trucks that drove by us. They definitely took that road.”

From "Finding Junie Kim" by Ellen Oh

Related Words

• precondition
• prerequisite
• requisite