Like a hard on, it comes about when some thing is arousing, but not enough to induce a full erection.
While watching two average women make out, Jim noticed that he was sporting a half on.
by Harry T August 14, 2007
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 September 06, 2005
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
by Its_Fucking_T0m June 14, 2011

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