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2.
Something said after denying, owning, or shutting someone down. The phrase comes from playing games after the game is over, the winner says "Thanks for playing" to the loser.
Jim: Yo dawg that bitch thought I was cute and gave me a free drink.

Tom: For real? She gave me the whole meal free.

Jim: ....

Tom: Thanks for playing.

After the 2008 NBA Finals Kevin Garnett and Paul Pierce turn to the Lakers and say:

THANKS FOR PLAYIN!
by ConsciousHipHop November 05, 2008
 
1.
The phrase "thanks for playing" is said arrogantly towards someone who you've just proved wrong.
Dustin: "Michael Turner led the league in rushing in 2008"

Craig: "That would be Adrian Peterson not Michael Turner...but Thanks For Playing!"
by cad281 August 29, 2009