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1.
the day of the week when you get blackout drunk and have sex with random strangers because you cant get pregnant.

please note: this does not make anyone a slut, cause when you cant remember it, it doesnt count!
Q-what are you doing tonight?
A-it casual sex friday what do you think
by founderofcasualsexfridays October 21, 2010
 
2.
casual sex had at the start of the weekend that doesn't involve any dressing up or fancy meals.
"God, my workplace is adopting that casual fridays thing. I'm going to have to see my boss in JEANS."

"Really? Let's go to a pub afterwards and get us some casual sex fridays."
by Tcakes February 21, 2010