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1.
When a threesome is about to occur between to men and a woman, the first man to yell out, "Butt-Gun" gets the "back seat" so to speak. The other man is stuck with the vagina. It is a way of claiming "pole position"

Etymology: Butt Gun is a derivative of the colloquialism "shot gun" used to claim the front passengers seat next to the window
Last night my friend and I met a lady at a bar. After enjoying a few hours of conversation and cocktails, we invited her to our room at the hotel for a movie and drinks. As soon as one of us took a chance and kissed her, I screamed out "Butt Gun !!!." My friend was immediately bummed out knowing I beat him to the punch. I was going to be the rear admiral.
by krismatt February 13, 2010
 
2.
A penis, in prison it is reffered as a buttgun. Shooting the semen into the butt of another male.
Bend over and let me show you my buttgun!
by gordon freeman 29999 November 28, 2011