What is the origin of 'bootleg' ('bootlegger', 'bootlegging'), in the general sense of "illicit trade in liquor" (OED)?
The Online Etymology Dictionary gives one possible origin, from 1889:
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.
Other possible origins include references to a tall beer glass, also known as a 'schuper':
Queer Drinking Vessels
The term "boot leg" applied to a very tall beer glass…may owe its origin to a misapprehension of the French touching on [an] old English drinking vessel. The black jack, a leather bottle sometimes lined with silver…According to a curious old book of the seventeenth century, when Frenchmen saw these vessels…they took back to France the story that the English drank out of their boots — N. Y. Sun.From The Iola Register (Iola, Kansas), 26 Jan 1894.
References to 'schupers' as 'boot-legs' appear at least as early as 1886:
Clipping from The Critic (Washington, District of Columbia), 28 Jan 1886.
Also prior to 1889 are mentions of 'boot legs' as an ingredient in the making of, for example, "tangle-foot" whiskey and "boot-leg coffee".
Best Answer
The New Oxford American Dictionary has: