The act of when a penis is half way erect, he slaps his penis across his partner's face.
when susan received a half mass from her husband, she was furious and bit off his nuts.
by Jen Sing June 30, 2006
Half Boner; When the penis is not fully erect but still hard with a softness.
BROOO, I have the biggest Half-Mass
by Warfared November 25, 2012