Many of you may be aware that Chabad (outside of Israel) pours a drop from almost every liquid before drinking.
This appears to be a custom of recent vintage, probably dating from the Geonic era or later, and is only commonly found among hasidim. The reason for the custom seems to be the contention that evil spirits dwell in the liquid, and pouring off a drop removes them. I've also heard it presented as follows: The poured-off drop appeases the evil spirits.
Judaism frowns on retaining customs that mimic idol worship. For example, that is why we no longer prostrate during prayer (except on Yom Kippur, and then only on covered floors, not directly on stone or cement).
What does this have to do with Chabad and yayin nesekh, wine of idolotry?
Enter the actual historical meaning of yayin nesekh:
Pouring some wine on the ground as a religious offering before drinking it was a common custom in the ancient Mediterranean world. In one of many such passages in "The Odyssey," for example, Homer relates that a character "mixed the honey-hearted wine and served it out to all, to each in turn. And they poured libations to the blessed gods, who hold broad heaven." For this reason, the rabbis of the Mishnah forbad Jews to drink wine produced, sold or served by gentiles, since a libation made from the same jug could implicate them in idol worship. Even if the chances of its being used for pagan purposes were remote, such wine was known as yayin nesekh or "libational wine." (In correct Hebrew, the term should be yeyn nesekh, being a genitive construct, but yayin nesekh is the way it is commonly said, even by the learned.)
However, the rabbis of the talmudic period, made an exception to this rule: If the wine was "cooked" by the gentile before serving, Jews could drink it, since "cooked wine" was never used for libations. Such "cooking" did not mean adding wine to a sauce, but rather heating it with spices to make what is known as "mulled wine" — a beverage that is consumed by English speakers today mostly as a traditional drink on Christmas and New Year's days. Mulled wine, legendarily invented as a winter restorative bù the physician Hippocrates, was highly popular in the Graeco-Roman world, in which it was heated over a wood or charcoal fire in an urnlike vessel called a calida. To this day it is available in winter in many Italian taverns.
By the Middle Ages, of course, wine was no longer being used in pagan libations, but Jewish religious conservatism kept the ban on yayin nesekh intact. Some medieval rabbis sought to justify this ban by extending it to stam yayin — Gentile wine even if was not "libational" — by arguing that drinking it might lead to the undesirable phenomenon of Jews and gentiles socializing together. While the same conservatism continued, rather illogically, to permit yayin mevushal, this was more a theoretical than a practical ruling, since most medieval Jews lived either in Muslim lands where wine was prohibited, or in Christian ones where winter pick-me-ups were made of stronger spirits. On the whole, medieval Jews made their own wine, although some of the less pious no doubt ignored the stam yayin prohibition entirely.
How ironic – and telling? – that Chabad promotes this pagan-like custom.