When someone tells an unbelievable story others in the room may secretly tellothers that the story is bull sh** by saying yellow shark. The teller of the story will have no idea that everyone knows the story is false if he does not know of the word's meaning.
Joe: Dude I totally got laid last night by two super models!
Steve: Lemon shark.
Frank: Totally a lemon shark.
Joe: What is a lemon shark?