What is the X in X-mas?
c.1300, probably from or related to Old Norse flengja "to flog," of uncertain origin. The Middle English intransitive sense is that suggested by phrase have a fling at "make a try." An obsolete word for "streetwalker, harlot" was fling-stink (1670s). Related: Flung; flinging.
"attempt, attack," early 14c.; see fling (v.). Sense of "period of indulgence on the eve of responsibilities" first attested 1827. Meaning "vigorous dance" (associated with the Scottish Highlands) is from 1806.