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1.
An expression used when something shocking happens, be it good or bad.
Man: Shit a duck! She is fucking hot!
___________________________________________

Man A: Mate I lost my phone yesterday
Man B: Aw shit a duck..
by Gurtek the Destroyer June 02, 2009
 
2.
the best way you can find to express your feelings when your really stoned and really have to pee
"wow..i really need to shit a duck."
by lambie November 22, 2006