A phrase used to indicate that the target is about to be introduced to a whole world of pain. Specifically, you, by opening this can, are going to unleash the power of whooping (that is, in the sense of attacking and beating, not in the sense of whooping cough) upon their sorry ass.
1. Specific use:
Jerry paused in thought.
"For what you have said about my mother, my girlfriend, and the proud ancestry of my forebears," he announced, "I have every intention of opening a can of whoopass."
2. General form:
"If <offending body> continues with <offending action>, it is totally time to open a can of whoopass"