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The Old McNulty is a sexual act in which at least one male or transvestite must be involved. It includes (but is not limited to) putting one's testicles in another individual's (male or female) mouth while ejecting feces on to their forehead.
A: So dude, what'd you do last night?

B: I was hanging out with this girl and guess what?

A: What?

B: I gave her the Old McNulty.

A: Oh, you ol' dog you.
by IronSpartan August 02, 2011
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