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When having anal sex, the milk-stirrer is the one who is putting their dick in the other person's ass. Picture the ass as the milk glass, the (eventual) semen as the milk, and the dick as the spoon stirring the chocolate (poo-poo) and the milk all up. A very sweet image, isn't it?
Bill: That was so awesome when Tyler stirred my milk yesterday.

Ted: Yes, Tyler is a very good milk-stirrer.

Bill: Go fuck yourself, bitch.

Ted: Okay.

*Ted takes out dildo and fucks himself*
by Manboobs are Cool April 03, 2011