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don't

[dohnt]
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verb
  1. contraction of do not.
  2. Nonstandard except in some dialects. contraction of does not.
noun
  1. don'ts, customs, rules, or regulations that forbid something: The boss has a long list of don'ts that you had better observe if you want a promotion.Compare do1(def 55).

Usage note

Don't is the standard contraction for do not. As a contraction for does not, don't first appeared in writing in the latter half of the 17th century, about the same time as the first written appearance of other contracted forms with not, like mayn't and can't. Don't remained the standard contraction for does not in both speech and writing through the 18th century. During the 19th century, under pressure from those who thought it illogical and who preferred doesn't in that use, don't for does not gradually became less frequent in writing but continued to be common in speech. Don't for does not still occurs in the informal speech and in the personal writing of many Americans, including the well educated, especially in the Midland and Southern dialects. It does not occur in edited writing or formal speech.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for don't

don't

contraction of
  1. do not
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 don't

contraction of do not, recorded from 1630s.

Online Etymology Dictionary, © 2010 Douglas Harper