A complete and utterly terrible handjob. One that is simply not worth the time and honestly makes one wonder what the fuck is wrong with with the human being who's head is in your lap.
Brady came back Jones' house and told everyone that Liz gave him the Slack-Jack of the century and he'd rather stick his dick in a pinecone than experience that again.
This is jive talking, and you shout out this when one of your homies busts a better move than you
"Yo man CUT ME SOME SLACK JACK that move was off the hook!
Slack-a-lack-jack
Means, roughly, "Please endeavor to give me the personal space that i desire while i'm buying, selling, distribting, or using crack cocaine." Old Phoenix (AZ) slang.
Be slack-a-lack-jack, ga'my pipe, ga'my crack!
the act of putting something off (usually skipping a class) to go masturbate
Friend 1: "dude where were you? there was a math test that was 90% of our grade!"
Friend 2: "Oh I was slacking and jacking"
Friend 1: "oh ok that makes sense"