This is an expression guys use to detemine if a girl is interested in you or if you are already drilling her? It can also be used as an expression for a hot piece of ass that one would like to nail.
Mike: Hey Joe, I saw you talking to Mary earlier today. Is she on tip?
Joe. Not yet, but I'm working on it.
As Joe and Mike are talikng, a gorgeous slut walks by. Mike says, "on tip!", then Joe says " i'd love to crush that pussy!"