Is there any other kind of nog besides egg? If EGG nog is the only kind of nog, why isn't it just called nog?
Seems that this is an interesting question, and there are a few different version of the story:
Either egg nog the term comes from egg + grog (slang for rum) or egg nog comes from egg beverage served in a noggin (small wooden cup) or egg nog comes from the british slang "nog" for stale strong ale...either way, it seems a bizarre concatenation of old words over time, hence the reason it's the only "nog" beverage there is.
For reference, these are the articles i found:
Thanks for the question!