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1.
an unnecessary phrase that precedes a story or a bragging statement.

most often used by people who are trying to sound mysterious/coy/cool, but it simply annoys the person they are talking to.

sometimes followed by "but definitely did" to make the phrase twice as useless.
Nerd: I may or may not have, but definitely did just solve the math problem first.
Normal Person: You're annoying, shut up.

Caitlin: I may or may not have met Miley Cyrus yesterday!
Mike: What do you mean 'may have'? Did you or didn't you?
Caitlin: I did!
Mike: Well you could've just said so.
by Polly Pissed-off-in-Physics November 20, 2011