bootleg (n.)

also boot-leg, "upper part of the leg of a boot," 1630s, from boot (n.1) + leg (n.). As an adjective in reference to illegal liquor, 1889, American English slang, from the trick of concealing a flask of liquor down the leg of a high boot. Before that the bootleg was the place to secret knives and pistols. Extended to unauthorized music recordings, etc., by 1957.