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1.
When stuck in a crowded, shoulder-to-shoulder, situation (ie. elevator, concert, Disneyland queue, etc.), you pull your arm inside your short-sleeved shirt and then stick only your exposed elbow through the sleeve. You then proceed to nonchalantly rub your stump on the uncomfortable stranger standing next to you.
While traveling in a crowded elevator in the Sears Tower to the observation deck, Larry proceeded to nubrub at least 4 people much to the delight of his giggling son.

Also see stumpbump
by JacknRochNY May 13, 2009
 
2.
When a woman rubs her clit on a man's (or woman's) nipple to achieve orgasm.
I did everything to this chick, but the only way she would get off was with a "nub rub!"
by Crickett6969 February 11, 2008
 
3.
A hand job following a massage, usually performed at an illicit massage parlor.
What a day at work! Time to get a nub rub.
by dutch passion April 30, 2003