The first time I ever heard the phrase the "angel's share," a friend was talking up a cooler-than-cool secret Japanese cocktail bar in New York of the same name. I mean, even that phrase holds some kind of street cred. It sounds deeply mysterious, as one would imagine—and hope—any interaction with a winged celestial being to be. What would angels be partaking in, anyway? When I remarked on this, my friend asked if I knew where the phrase came from. Obviously, I had no idea. So he went on to explain the near-magical origins of "the angel's share." In a word? Booze.
And in a few words: Barrel-aged booze. When a wine or spirit is aged in a barrel or cask, some of the liquid evaporates. What's lost becomes known as "the angel's share"—likely because it disappears skyward, almost like an offering. The amount that evaporates is dependent on a range of factors, though, including the climate the alcohol is stored in, the size of the cask, the storage technique, and the age of the spirit.
Hotter climates will see alcohol evaporate more quickly. In drier climates, more water evaporates, strengthening the concentration of alcohol. Conversely, in more humid climates, more alcohol will evaporate, while more water stays behind, lessening the amount of alcohol.
Cask size also makes a difference. Booze stored in smaller casks evaporate more because there's more wood-to-liquid contact. This also speeds up maturation times. In addition to this, airflow makes a difference. So, if casks are stored on racks or are raised above the ground at all, air circulates all around them, also encouraging evaporation.
Finally, when an alcohol is "young"—or hasn't been aged at all—evaporation happens more quickly. Glenlivet, for example, says that when their whiskey is first in a cask, it loses about 3.5% to 4% of its alcohol in a year. Over time though, it'll average about 2% evaporation. This can take a toll though: after a 20-year-long aging process, a whiskey can lose 40% of it's total volume.
Interestingly, in the world of wood barrels, the devil gets an offering, too. "The devil's cut" is how distillers refer to the amount of alcohol that's been lost to absorption by the barrel itself.