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1.
An abbreviation for "day of the month", in reference to the time of the month when a woman has her period, often resulting in increased emotiveness (and often noticeable bitchiness) towards those around her.

See also TOTM and PMS.
John: Hi Mary!
Mary: WHAT IS YOUR DAMN PROBLEM?

John: But Mary, I just said hello.
Mary: Sorry. DOTM.
by Noam Chomskie December 09, 2010