Someone, usually a girl, used to make "boo-boo's" better, then thrown away and forgotten abt til something else comes along and they're needed again. And usually never get anything in return, or helped when needed.
<Friend 1> Your friend just broke up w/ me, can you talk to her for me?
<Friend 2> Yeah sure.
2 days later
<Friend 1> Hey, can I borrow some money?
<Friend 2> I'm sorry, I don't have anything on me...
<Friend 1> Whatever! Did you talk to your friend?
<Friend 3> She's such a band aid