Whup-Ass is OFFENSIVE while ASS Whippin' is DEFENSIVE. When someone says, "I am going to WHUP your ASS" they are on the offense. Getting your Ass Whipped means being on the receiving end of Whup Ass.

I couldn't help but notice this very precise definition of "Whup ass and Ass whipping. It made me wonder if these definitions come from Wiki or what?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.