(n) A
person, usually female, who is
dating your best friend. Whenever you throw a party or just
casually hang out, they are automatically invited to tag along because coincidentally, they are equally awesome.
Richie: "Hey buddy, are you going to watch the football games this
afternoon?"
Tim: "Yeah, there are some good ones on today. I got my chips and
dip, pizza and
soda ready to go."
Richie: "Cool story, bro. Maybe I'll stop over later. Mind if I bring Ilsa?"
Tim: "Yeah sure. She's always welcome. Now that you two are together, she's a Bro By Association.