Meaning

noun a punch made of sweetened milk or cream mixed with eggs and usually alcoholic liquor

originAmerican English c. 1775 (although drinks like it are attested far earlier, e.g. posset, of which a monastic Christmas recipe used eggs), from egg + nog. The second element is uncertain and long-debated; prominent claims include Norfolk dialect nog (“strong ale”), or a clipping of noggin (“small, carved wooden mug used to serve alcohol”) (q.v.). Another hypothesis is that it is a variant of grog…

Etymology from Wiktionary, CC BY-SA 3.0