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1.
To grip or squeeze the buttocks of a member of the opposite gender, (or the same gender if that's what frys your bacon) thus allowing the butt cheek to come to rest momentarily in the hand.

Duration and pressure of hand contact on the buttock generally denotes the amount of sexing desired by the palmer.
I was at a party and this blonde science girl was totally checking out my package, so when I walked over to refill my drink, I gave her an nice subtle ass-palm to let her know I was interested in sexing her up later on that evening.
by peafarter July 25, 2003
 
2.
comes from the action, facepalm (see definition). 1) comes after someone facepalms 2) Asspalming is when you smack your hand on to someone's ass, full palm grab.
1)

James: touching my hair is a big no no
Jez: *touches James' hair with her fingertips and runs away*
James: *facepalms*
Dee: *asspalms*

2) Toy: *asspalms D*
by Ms. Ducky Dee August 31, 2009