When a girl bends over and her man puts his penis in her ass crack without any penetration. This is usually done as foreplay before [doggystyle] sex or as an act when the man and the woman do not want to engage in full intercourse. It is the ass equivalant of a [titty fuck]. The womans ass cheeks are used to excite the penis, sometimes to ejaculation. Her ass is polishing your tip.
ALSO, a girl who has a nice ass. This is because a girl needs a nice ass in order for this act to be pleasureable.
Many people confuse this word for the equivelent of running the train. This is because this act is used by many men on one woman on after the other. This is common because a tip drill does not involve penetration, making it easier to be repeated on one girl in a short amount of time.
The act is mock performed in Nellys Tip Drill video. Also, the video focuses on the ass crack and at the end, Nelly swipes a credit card on a girls ass, simulating the act.