To utterly and totally destroy the punchline of a joke so as to make it completely devoid of any sort of humerous value. Often, the act of CantonING (the verb form) is funny, because the person who catoned the joke often is not aware they have done so.
This term comes from the town of Canton in Central Illinois where people have a tendancy, due to life in such a provincial area, to miss high-brow humor and explain simple jokes with what they see as...jokes.
"He totally Cantoned that joke."
Example of Cantoning:
person "A": "This room smells like 15 kinds of ass."
person "B": "Yeah, almost like 20 kinds of ass!"
Person "A": "What's green and flies?"
Person "B": "Superpickle!"
Canton person: "Yeah, hahaha, or super green bell pepper."
*everyone pauses and stares, looking awkwardly at one another*
Canton person: "Sorry, I work at Hy-Vee."