When a bunch of questionable dudes each take a paper plate, go upstairs, cut a hole in said plate, work one up, put their respective cocks in said holes, go back downstairs, and present before an appropriate panel of judges (panel is made up of fellow dudes, not gals).
Cockoffs can also take place in a dudes backyard....perhaps at some sort of celebratory cook-out.
Weasel: Blake......I guarantee I have a larger COCK than you.
Blake: With utmost heartfelt respect, I disagree.
Weasel: Why don't we cockoff then?
Blake: I'll go get the brand new ream of plates I just bought.