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1 definition by Alex Donaldson

 
1.
n 1: The monthly recurring date of a past event, especially one of historical, national, or personal importance: a relationship monthaversary (monthiversary); the monthaversary of ones birthday (bithday).

2: A celebration commemorating such a date.

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n: the date on which an event occurred in some previous month (or the celebration of it)

In dumb down stupid terms: A monthly anniversary, though usually only used in context for the first 12 months
Today is the 2nd monthaversary of that nasty hurricane that hit two months ago
by Alex Donaldson October 15, 2005