When you're with one of your boys and you're trying to catch some tail for the night via text. You eventually convince one girl to meet up but still need to watch out for your friend, so you get the girl to bring one of her friends to low-key hook up with your friend. Her friend is referred to as Mystery Friend because neither of you know her name, or more importantly, her looks. Mystery Friend is often far less good-looking than the original friend, usually fat, and always desperate.
Mike: Yo! I got Porschia to hang out tonight and she said she'll bring a friend for you!
Jon: Nice! Is she hot?
Mike: No clue.

Jon: Oh great, a Mystery Friend...
Mike: Just plan on the Mystery Friend being a 4 and hopefully you won't be disappointed when you see her.
by FlowJo March 1, 2017
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New friend of one of your maybe gaybe buddy.
"Did Joe bring a date?" "Nah, he was with one of his Mysterious Male Friends."

"Dude, ran into your buddy Steve. Wearing guyliner. With a Mysterious Male Friend."
by kellybex July 20, 2009
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