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1. Half-On
A semi-erect penis. (See: Semi, Chub) Fifty percent of a Hard-On. A "bulge in the pants," as it were.
I often get a half-on while leering at females on the subway.
by Time4SumAksion Sep 6, 2005 add a video
2. Half-on
When a mans dick gets bigger but not hard enough yet to not be bendable either with starting to get a boner or starting to go down.
Damn that guy has a huge dick

Nah he just has a half-on
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