-To quack is to perform an act that displays one's stupidity, incoherence, lack of coordination, or outright inability to relate or adapt in a social setting. Derived from the sound a duck makes, more specifically the verb form for what a duck (see definitions 5,8, and 9 for "duck") does.
-To fuck up the situation for everybody.
You missed the biggest quack!
____ ran down the hill, tripped and busted ass all over the sidewalk and fence by the tennis courts!
How was your birthday?
Fun as shit, we got fucked up then ______ quacked ridiculously by hitting the blunt BACKWARDS! He didn't even notice for like 2 seconds, then he got up and slid down the hill and lost his keys.
What a fuckin' quack
I dropped the entire pizza on the ground.
You're so quackin.