Top Definition
The act of shoving your testicles into the anus of a girl you are having vaginal intercourse with in the missionary position.

May also include shoving your testicles into the vagina while having anal sex.

Akin to an "Armstrong".
"Dude, I was doing this chick and I totally MacMillan'd her. Right in the backseat of my Miata!"
by TroubledIndividual March 26, 2009
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