minhag - Dipping fingers in Havdalah Wine

What is the source or origin of the custom to dip one's fingers in the wine poured over the candle to extinguish it at the conclusion of Havdalah?

As a side note, and a reason I'm tagging it this way, is that I always joke that it's 'Avodah Zarah (often while actually doing it). It's not ... is it?

Answer

Rema, Orach Chaim 296:1:

גם שופכים מן הכוס לאחר הבדלה ומכבין בו הנר ורוחצים בו עיניו משום חבוב מצוה

"We also pour out [some of the wine] from the cup after havdalah, and extinguish the candle with it, and wash our eyes with it as an expression of love for the mitzvah."

An earlier source is Tur, Orach Chaim 299, citing R. Amram Gaon. So it goes back a long way.