When you go to reach for something from a chick and they move and you "accidentally" grab their boob. This also works with really fat guys.
Nathan: Hey Kim, can I see that picture you're holding?
Kim: Yeah, sure, you just have to reach in front of my REALLY BIG BOOBS to get it.
Nathan: Okay! (a little too enthusiastically) Whoops, I that was an "accidental" boobgrab.
kim: Well... it looks like you're gonna have to try again!
*Some Random Person: Crap, I just boobgrabbed Peter Griffin!*
To grab someones boobs. An adverd which comes from the verb "to grab" and the noun "a boob."
I performed a boobgrab on your mama.