Half-jack Share definition Flag You get so uninterested in masturbating, that you stop before you cum. Peter: Dude! I was so baked last night, I was whacking off and got bored and stopped.Paul: Just a half-jack then? Half-jack by Goldfish408 March 4, 2010 0 0 Get merch Get the Half-jack mug. Buy Now — $32.95 See more merch