1
general::
Under no condition, not at all, as in In a pig's eye he'll pay me back, or You think he's competent? In a pig's eye! This expression, a euphemism for in a pig's ass, is generally used as a strong negative. [Slang; late 1800s]
American Heritage Idioms
2
general::
Phrase(s): In a pig’s eye! [and] In a pig’s ass!; In a pig’s ear!
Rur. Nonsense! (Use caution with ass.) • Tom: I wasn’t going to steal it. I was just looking at it. Jane: In a pig’s eye! I saw you put it in your pocket! • Mary: Bill says he’s sorry and he’ll never yell at me again if I take him back. Jane: In a pig’s ass! He’s made those promises a hundred times before. • Tom: I thought you said I could keep this. Charlie: In a pig’s ear! I said you could borrow it.
McGrawhill's American Idioms And Phrasal Verbs