A-Half-hearted response to a question that isn't important.
B-Sarcastic response to a serious question.
C-The answer given when asked a question you don't know the answer to. Ususally spoken as a question.
Example for def. A-
Person A- "What are you doing tonight?"
Person B- "My left ass cheek."
Example for def. B-
A- "Where did my purse/keys/wallet go?"
B- "My left ass cheek."
Example for def. C-
A- "Billy, what is the Capital of Zimbabwe?"
b "My left ass cheek?"