Etymology I Did Not Know

yule (n.)
...from Old Norse jol (plural), a heathen feast, later taken over by Christianity, of unknown origin.

That's the trouble with the study of word origins - as a rule they only go back so far. Sometimes they will go so far as to a suggest a root in proto-Indo-European or something for which we have no written record. But, even if that's right, what came before that? Something surely did.

Etymology ends in mist
For every word that ever was,
Except for the murmuring list
Of words like "buzz".