Mental Floss has an excellent, brief grammar lesson about the finer and arcane points of English syntax frozen as it were in the lines of traditional Christmas songs. It was certainly a fun and lively read and causes one to think of other examples, quirky little conventions that reveal how language evolves.
The etymological curiosity in the word troll, sometimes sung as toll or trawl, is especially interesting, as it reflects both Anglo-Saxon roots and the more familiar mage in later Norse influences. In the sense of the carol, it reflects Old English origins, prior to the arrival of the Vikings, to go about or to stroll. The connection with fishing, a drag-net, also extends from this source. The sense of a monstrous creature has old Germanic roots and though the English had their own words for native orcs and demons, they borrowed the word of the newcomer. Perhaps the two meanings again converge in the ultimate sense of a horrid individual who is trawling for attention with nasty comments. English did borrow a lot of basic vocabulary from the Scandinavian languages, and interestingly what’s been retained of—or edged out by—Norse terminology are words with an overwhelmingly negative connotations, which probably bespeaks their uneasy cohabitation: anger, awkward, blunder, bug, crook, cur, death, dirt, dregs, gawk, heathen, Hell, irk, mire, muck, muggy, odd, outlaw, rotten, skull, slaughter, thwart, ugly, weak and wrong—to name a few. Of course, there are numerous exceptions, too—like that word Yule, for the midwinter months and associated festivities, which was later appropriated by the Christians. 
