The male sex organ (And no, I don't mean the asshole, that is not at all a sex organ. Just because homosexuals use it in intercourse, wouldn't mean that a fist is a sex organ used by nerds for masturbation)
Also the unit used in measuring a boy's hanging fucking cock. = 1 Fingernail of King Henry I.
13 boy units = 1 man unit = you are a man
(Woman units are merely cup-sizes. It is hard to give the down-below tightness a measurement and a unit.)
Final meaning: a male person who wastes time looking down other peoples newspapers at their crotch area if applicable.
A girl's favorite toy often becomes a boy unit.
Man did you see the cock on that guy! I barely did and that makes it worse than if I didn't. Must be half a boy unit.
God stop staring! Why are you such a boy unit, you pervert. Mom! - sister complains about brother.