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When you sneak up on someone and yank their pants down so bad their junk is showing, and if you're extra mean, you also yank their underwear down too. It's the ultimate way to make someone look like a fool in front of everyone.
My buddy got dacked in front of the whole class and couldn't stop blushing for the rest of the day.
I dacked my teacher when he gave me detention and it was the best day ever.
At the party, my cousin got dacked by his friend and had to leave the room laughing.