Top definition
1. When you give a girl laxatives then you butt fuck her and she poops on your dick and you slap her in the face with your shit covered penis

Guy 1: Dude, PJ and Ginna did a Montana Milkshake at my grandma's house

Guy 2: Dude, thats disgusting

Guy 1: Yea, but Pj said it felt kinda good
by Urban Chaos May 14, 2007
Happy St. Patties Day!
Close cousin to the Cincinnati Juice Box, except the man releases a load of ejaculate into the woman's anus and proceeds to slurp the creamy mixture out with a bendy straw.
Dude, I totally gave Kevin's mom a Montana Milkshake. Mmmm Mmmm Good.
by Feefer May 16, 2008
Happy St. Patties Day!