In attempting to dissect "stompclacker" (Using thick gloves and very long tools. He's a wily one, that bilby.) I came across the following document which purports to be a glossary of informal medical terms current in Yorkshire. It's good for some chuckles.
I think our resident marsupial misconstrues the meaning of "whoop" in the countrified American expression "open a can of whupass.". It is not a celebratory cry but rather means a whipping or beating. So, one trash-talking basketball player might say to his adversary, "I'm gonna whup your ass!" To open a can of whupass is to invite calamity. It is a more local and limited version of opening Pandora's box.
I find the key syllables spelled as an unhyphenated "whoopass," hyphenated as "whoop-ass," and as two words - "whoop ass." The first syllable may be whoop, woop, wup, or whup. The most common version I find is "whupass."
A more exact British version might be, "open a tin of thrashbottom." The trouble with this formulation, however, is that the threat may not be received as entirely unwelcome. We colonials hear stories of the widespread plying of the cane on tender young bottoms in the "public" schools and of a fondness for such "correction" that persists into adulthood. Would an English cricketer intimidate his opponent by pledging to thrash his bottom or would he make a new friend?
Perhaps bilby could lay a long ear to the ground and provide us with an Australian equivalent.