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1.
The term used to describe masturbating in a porta-shitter while in the field.
PVT 1: "Dude, we need to go back to the rear so I can get laid!"

PVT 2: "Why don't you just go to the shitter and take a knuckledump?
by SGT JPB December 17, 2010
 
2.
Knuckle Dump:

Ejaculation through masturbation.
This girl was so dirty she asked for the old knuckle dump on the chin. The circus is in town people and there WILL be face painting.
by Kuric May 01, 2008