When you store an object or objects behind your knee, whilst lifting the apropriate leg aloft to keep said object(s) in place.
"Yeh, i didn't have any hands free, so i used my Knee-Bag for a bit"
by Phatang May 12, 2009
a tea bagging that occurs on one's knee, as opposed to one's mouth in a regular tea bag
Madison knee bagged Chase today for no apparent reason
by superwaffles March 06, 2010