Some Christmas carols have been around so long that they use beautiful archaic language like "hither and thither," which most English speakers can still recognize as meaning "here and there." But I bet even native English speakers don't know the meaning of some of these words that pop up in their favorite Christmas songs.
Lowing
Heard in: Away in a Manger
“The cattle are lowing, the poor baby wakes”
Lowing (verb) = Mooing
Verily, RivenHeard in: Ding Dong Merrily on High
“Ding dong, verily the sky is riv’n with angel singing”
Verily (adverb) = Truly
Riven (adjective) = Split
BobtailsHeard in: Jingle Bells
“Bells on bobtails ring, making spirits bright”
Bobtail (noun) = An animal that has had its tail cropped, usually a horse, dog or sheep. In this case, probably a horse with bells on its harness.
IncarnateHeard in: Hark! The Herald Angels Sing
“Veiled in flesh the Godhead see, hail the incarnate deity”
Incarnate (adjective) = Embodied or represented in human form
LeagueHeard in: Good King Wenceslas
“Sire, he lives a good league hence underneath the mountain”
League (noun) = a measure of distance, equal to about 3 miles
Deck, TrollHeard in: Deck the Halls
“Deck the halls with boughs of holly … Troll the ancient Yuletide carol”
Deck (verb) = Decorate
Troll (verb) = Sing
HeelHeard in: You're a Mean One Mr. Grinch
“You’re a mean one, Mr. Grinch, you really are a heel”
Heel (noun) = an inconsiderate person
Once bitten, twice shyHeard in: Last Christmas
“Once bitten and twice shy, I keep my distance”
Once bitten, twice shy (idiom) = Someone who has a bad experience will be more cautious next time
Thanks to Facebook fan Nelly for this suggestion!
WassailingHeard in: The Wassail Song (Here We Come A-Wassailing)
“Here we come a-wassailing among the leaves so green”
Wassailing (noun) = Caroling, Going door-to-door
Auld lang syneHeard in: Auld Lang Syne
“Should auld acquaintance be forgot and auld lang syne?” - Auld Lang Syne
Auld lang syne (phrase) = old times, times gone by (This is actually a Scots phrase, not an English one, but it’s sung every year around this time in the English-speaking world)