This is something that you say when someone passes gas audibly. It refers to the similarity of a farting sound to the quacking sound made by a duck.

In some cases, this expression is used by the farter to comically deflect the embarassment. In other cases, it is used by a bystander to comically point out the fart to the general public.

(noun) A successful type of theft which results in an acceptable, impressive and rewarding payday for the protagonist.