a disagreeable or unpleasant day, especially one during which one feels unattractive.
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British Dictionary definitions for bad hair day
a day on which one's hair is untidy and unmanageable
a day of mishaps and general irritation
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
Other Idioms and Phrases with bad hair day
A day when one's appearance, especially one's hair, does not look attractive. For example, What have I done to upset Martha? Nothing, she's just having a bad hair day. Originating as a humorous statement, this term was soon broadened to mean simply having a bad day, that is, a day when everything seems to go wrong. [Late 1980s] Also see not one's day.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.