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.
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