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1.
ass pumping - utilization of the buttocks in a general up and down motion to facilitate the use of a pump or other similar applied pressure working device. Definition may be inclusive of fully or partially clothed buttocks, or entirely unclothed buttocks.
Common devices used when ass pumping:
tire pump, beer keg "tap"

Expressions: "You can pump a keg with that ass!"
by TCR May 15, 2006