Green’s Dictionary of Slang

pig’s arse! excl.

also pig arse! pigs! pig’s body! ...bum! ...ear! ...tit!
[arse n. (1)/ass n. (2)/bum n.1 (1)/tit n.2 (1)]

[1910s+] a contemptuous excl., often negating a previous statement.

In exclamations

in a pig’s arse! (also in a pig’s! in a pig’s ass(hole)! ...butt! ...ear! ...eye! ...gizzard! ...hole! ...kapooch! ...knuckle! ...navel! ...neck! ...patrot! ...pink pyjamas! ...poke! ...prick! ...rump! ...snout! ...tonsil! ...valise! ...whinny! in the pig’s...) [US rural catchphrase in the pig’s ass, referring to bestiality and, as such, the subject of a variety of coarse jokes, which depend on the mistaken orifice (the anus rather than the vagina) and the mistaken object of affection (the pig rather than the woman); arse n. (1)/butt n.1 (1a)/hole n.1 (1a)/prick n. (1)]

[mid-19C+] (orig. US) a disdainful, excl. of negation; completely impossible, absolutely not! I don’t believe you! go away! also as adv.