The myth says that the expression “a boot of ale” comes from the custom of cutting off the top of old boots and using them as serving containers. (How the top of an old boot transformed into a vessel is unclear–to me, at least.)
As most of you who work at or visit colonial-era museums know, the American colonists drank out of leather vessels called jacks or blackjacks. These were lined with pitch to make them waterproof and are very sturdy. Decades ago, my stores in Colonial Williamsburg sold reproductions, and I believe they still do. These large leather jugs and mugs made such an impression on the French visitors to England in the 17th century that they reported that Englishmen drank out of their boots! A funny story, not meant to be taken literally. (Waterer’s Leather in Life, Art, and Industry, 1946, London)
Why use leather to make a drinking vessel? It’s an English tradition. In medieval England, there was little glass manufacture, so aside from wood, pottery, or tin, what are you going to use to make a mug or goblet? (Yes, gold and silver, but those are for the nobles, not taverns or average folks, so let’s not go there.) Leather worked very well. Still does. But not boots.
Comments:
David Lloyd 
Walkowski Lee 
History Myths Debunked 
Can you tell us about bootlegging, then? It must come from the same origin. I can’t imagine it’s a reference to carrying liquor in the boot of one’s car. I’m reading your novel, so it’s on my mind! Well, actually I’m listening to the audiobook from Audible, but I still call that “reading”.
The word first appeared in the 1850s in Maine and of course it refers to smuggling liquor. But this seemed odd to me because Prohibition didn’t start until almost 70 years later. That is, except in Maine, the first dry state, where it became illegal to manufacture or consume liquor in 1851. Because Maine shares a border with Canada, the law was easily flouted. Ordinary folks wanting to smuggle liquor into the country could hide a couple bottles in their pants legs in Canada and walk into the United States.
It’s not about the ‘boot’ of a car, but a literal boot, where flasks of whiskey would be hidden and carried across the border. I live in one of the major cities known for it’s role in the prohibition.
Looking at your illustration/example above, you can’t blame anyone for jumping to a conclusion that people were drinking out of their boots! Except for the stitched handle, that example looks very much like an inverted riding boot. Perhaps they were made from the same leather stock, and from a similar pattern.