Open A Can Of Whoop Ass Meaning

(idiomatic, slang, US) A good-humored threat of physical harm.

Example: I'm gonna open up a can of whoop-ass on ya.
1998, Robert Fletcher, No rush to see new Chan, University Wire
  Chan stars in his first all-American production as chan as Hong Kong Detective Inspector Lee, a noble, innocent and caring police officer who can really bust open a can of whoop-ass when given the chance.