When a man, gay or otherwise, has anal sex with at least 3 or 4 other men throughout the same day and night. Girls can be count as part of the mud run, but the one making the run is always a guy.
Guy 1: You guys wanna come over this weekend for a BBQ, some drinks and then maybe a mud run.

Guy 2: Ah, yeah, that could be fun. I've never done a mud run before.

Guy 1: You don't know what you're missing. You can even bring a couple of girls along, just not too many.
by small mammal May 13, 2013

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