Definition from Wiktionary, the free dictionary
- (slang) (vulgar) Indicates disapproval, disagreement, or disbelief, often with a tone of disregard, disdain, or disgust.
- She'll use my money to buy a new car, my ass!
- You're his brother, my ass! You look nothing alike.
See also 
Indicates disapproval, disagreement, or disbelief
my ass (personal pronoun; the first person, singular)
- (slang, vulgar) I, myself; oneself in the third person.
- My ass is tired.
- Now I gotta drag my ass to work.
Usage notes 
- Sometimes shortened to m'ass.