Green’s Dictionary of Slang

an excl. used to imply one’s contemptuous rejection of the previous speaker’s assertion; sometimes as your foot!

1905		Sporting Times 1 Apr. 11/2: Said No. 2: ‘My hat! this is a really nice girl.’ Said No. 1: ‘She is a nice girl, old chap, but that was my foot.’.
1911		G. Stratton-Porter Harvester 515: ‘Grandmother your foot!’ cried the old woman.
1922		Wodehouse Clicking of Cuthbert 135: Pleasant morning ramble my number nine foot!
1937		R. Westerby Wide Boys Never Work (1938) 72: Liability my foot...
1940		F.S. Fitzgerald ‘Pat Hobby’s Christmas Wish’ in Pat Hobby Stories (1967) 32: ‘My foot!’ said Harry contemptuously.
1947		G. Fairlie Bulldog Drummond Stands Fast 70: ‘Her father, my foot!’ grinned Drummond.
a.1948		(con. 1930s) C. McKay Harlem Glory (1990) 24: Baron, my foot!
1958		H.E. Bates Darling Buds of May (1985) 34: Pason’s Poke, my foot. Sharp as a packet o’ pins.
1968		Dandy 30 Nov. n.p.: Ghost my foot!
1974		D. Ireland Burn 60: ‘Secret me fat foot,’ McAllister says.
1989		R. Dahl Rhyme Stew (1990) 17: A mouse my foot! It was a HAND!
1999		K. Sampson Powder 70: The Tweets outselling Oasis, my foot!
2006		W. Chen Chutney Power and Stories 120: ‘But Madam, it surely slipped from my hand’. [...] ‘Slipped my foot, my big foot!’.