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1.
The act of trying to hide an erection by tucking your penis into the waistband of your pants. This method is usually used when in public and/or wearing loose fitting pants or shorts.
I got an erection while staring at that woman but i was in public so I had to tuck up.
by Dertty December 30, 2005
 
2.
To shaft or prank someone, usually a mate. As seen on some of the WKD adverts
You tuck up a mate, when they can't get away from someone talking to them and you don't rescue them.
by richard jacobs February 05, 2007