If you look at the entomology of most languages, grammar usually involves shortening of longer phrases because people are lazy:
I am -> I’m
The stars that are shining… -> the shining stars
So, the phrase “whup ass” is most likely a shortening of the common expression “…will whup his/her/their ass.” The “can” part is likely to be a way to express intensity or pluralize it. Again, the theory of lazy grammar states that “can of whup ass” will more likely be used that “a six pack of whupass” or “a dozen extra large grade A helpings of whupass” or “a spoonful of whupass” (Will and Grace.)
So, while the origin of the phrase could have been done randomly at any time “whup” first met “ass,” it wasn’t popularized until “Stone Cold” Steve Austin used it as a catch phrase and started selling t-shirts with that on it, thus standardizing the plural/intense form.
Edit: “Intense form” usually is the more/most/-er/-est forms, although “he has the potential to be the wuppiest whupass whupasser there ever was” does deserve to enter the language.