Green’s Dictionary of Slang

1. an unlicensed drinking house, a speakeasy, an ‘after-hours’ bar.

1953

J.P. Roppolo ‘“Blue”: Indecent, Obscene’ in AS XXVIII:1 12: A blue pig is an establishment for the sale of illicit liquor.

2. the whisky served in such an establishment.

1934		Weseen Dict. Amer. Sl. 273: blue pig, whisky.
1948		C.G. Laird Laird’s Promptory 936: [Liquor:] slang: firewater, blue pig, John Barleycorn, redeye, rotgut, snake medicine, white mule, mountain dew.
2002		Seadog Guildbook ‘Cat Howell’s Book of Naughty Nomenclature’ 🌐 Booze: [...] Blue pig Blue ruin Blue stone.