Buttbro is when the person in question, is being buttbroed in the butt region by a "bro" (Mostly a friend at the same age).
Mostly taken place in private school all over the world.
1. Lets do some buttbroing?
2. Have you seen my buttbro?
3. Hi buttbro.
4. Get mah gun, there's buttbroes on the corn field doing buttstuff...