The Weekly Walla Walla Wine Word for Dummies is: Angel's Share
The name itself refers to the portion of wine that naturally evaporates through the porous oak barrels during the aging process.
Centuries ago the monasteries were noticing this natural process happening in their cellars. Since there was no real scientific explanation at the time, and especially since no monk wanted to accuse his brother of sneaking sips, it was determined that the loss of the wine in the barrels signified something heavenly and they were making a lot of angels happier - a divine intervention, no doubt. So the name, "Angel's Share" arrived.
There is also a different term for the evaporation in a barrel mostly to do with whiskey. It is known
as the "Devil's Cut." So depending on what team you root for, angel's share or devil's cut, just don't blame it on the brother monks.