Top Definition
When you decide to blow off all responsibilities on any given sunday including work or going home if you've been out of town partying all weekend. Very popular term used in the rugby community. Should always involve hanging out with members of the opposite sex and is even better when you actually have butt sex with one of them.
Skip: Well, should we head back to the twin cities, I have to work later.

Joe: Screw that, we're in Chi-town, it's Butt Sex Sunday, all day long.
by bretty b December 13, 2009
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