According to the renowned urban dictionary:
milk shake
22 votes
the sexual act of a male putting his penis between a female's breasts, and thrusting his pelvis back and forth.
usage: Last night, I met this girl with the biggest boobs, and I milk shaked her.
So now not only does this song suck, but it makes no sense as well
EDIT - I always thought this was "Angel Fucking" :/