Green’s Dictionary of Slang

(orig. US) an excl. used as a retort to an insult or slur.

1926		Wood & Goddard Dict. Amer. Sl. 52: So’s your old man. An expletive, a snappy retort.
1928		M.C. Sharpe Chicago May (1929) 108: The English had a slang expression, ‘There’s hair,’ similar to our present American slang, ‘So’s your old man,’ or ‘For crying out loud.’.
1943		Baker ‘Influence of Amer. Sl. on Aus.’ in AS XVIII:4 256: Here is a representative group of Americanisms which have wide currency in Australia: [...] so’s your old man.
1960, 1975		Wentworth & Flexner DAS 504/1: So’s your old man An expresion used in scornful or contemptuous reply to any remark that arouses disbelief Orig. West Coast use, c.1915.
1967		G. Hellman Smithsonian Octopus on the Mall 131: The bird could only say [...] ‘It’s been raining all day’ and ‘So’s your old man’.
1985		D. Curley Living with Snakes 45: To his astonishment the paper on which he had carefully printed his message to the world had been reversed, and he now read: ‘So’s your old man’.
2000		R. Berton (con. c.1930) Remembering Bix 274: We passed cars coming the other way and yelled nonsense at them — ‘Whoopee!’ ‘So’s your old man!’ ‘So’s your Aunt Mabel!’ ‘So’s your anchovy!’.