Abbreviated form of "vomit in my mouth", used to express disgust, generally at a suggested activity. The "M" can be swapped out for other readily identifiable words, such as 'C' for car.
"Hey, wanna go eat at Applebees?"
"Oh, V in my M. Lets go to Chili's instead."
"She's really drunk, we should give her a ride home, I just hope she doesn't V in my C."