Like a hard on, it comes about when some thing is arousing, but not enough to induce a full erection.
de Harry T 14 août 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.
de Time4SumAksion 6 septembre 2005