also known as "the chubs", a half boner is semi erection (technically hemi erection) of the penis in males. It is often obtained when a man's feelings are on the fence and he is not sure if he is in love with someone or not. It is the female's job to cause the half-boner to either extend into a full one or retreat back into it's phallic lair. This is accomplished by said female "working it" (work it) and showing the man what she's got. If she can't make a boner, then there is something unappealing about her and she will probably never have children as her life will be reduced to one night stands with men in bars (see: Beer Goggles) as they will be the only ones who can get a full boner until they wake up next to her in the morning.
Also: see Chubs as a synonym, there is a little song that goes with this definition:
I've got a half boner for you
Big fat chubs for you
Oh how happy you'll be when I get this lumpy chub out of my jeans
But the shape should let you know - you got a little ways to go.
But if you gotta get to town, you can do it with a tiny mound.
Guess nobody will know you got the lumpy chubby stump on the go.
When your lady doesn't get you off, at least you'll have a package that's semi-soft.
In a sentence: That chick has a rockin' body, but I can only acheive a half boner because her ugly face (see: Butterface) is so distracting.
The stage of the male erection between flacid and fully aroused.
Yo, that female's cammel toe gave me a half boner.