When you and your friend (usually a male) are walking or standing next to each other and a female friend of his (or her's) stops walks towards you and unintentionally puts their breast onto your arm, usually occurring in a horizontal position.
(Guy 1 and 2 are standing next to each other)
Guy 1. So as I was saying is that report due tomorrow?
Guy 2. No she said it was due on Monday.
Girl 1. Hi John! (Walks over to both of them and hugs Guy 1.)
Guy 2. Dude she just gave me boob-arm.
Guy 1. I know.