A semi-sexual act, more often than not considered silly. Mickey Mousing is the act of a large breasted woman setting her breasts on top of the head of another person. Essentially, the breasts settle on either side of the partner's head, looking like Mickey Mouse ears.
Alternatively, it can be called Mickey Moused, in the past tense.
1 - Dude, she's got her tits out and she put them on this chick's head!
2 - You mean she Mickey Mousing her!?