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An educational way to say you didn't by the use of new-frontier, questionable grammar. Without the contraction, reads "must have not."
I do not recall, so you must haven't.

I must haven't heard about the band, probably because they're only on Myspace and suck ass.
by Experimental Mistress October 12, 2009
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Apr 21 Word of the Day
I've been jabbed
by Francesco II March 20, 2021
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