On-call skeezer
A girl (usually on the slutty side) that you can reasonably rely upon to hook up with whenever you call her, regardless of the time.
M1: I heard you booty called Monica last night. Was that a one-shot deal?
M2: Nah, she's been my on-call skeezer for a while now.
M2: Nah, she's been my on-call skeezer for a while now.