What is the X in X-mas?
Old English walhnutu "nut of the walnut tree," literally "foreign nut," from wealh "foreign" (see Welsh) + hnutu (see nut). Cf. Old Norse valhnot, Middle Low German walnut, Middle Dutch walnote, Dutch walnoot, German Walnuß, So called because it was introduced from Gaul and Italy, distinguishing it from the native hazel nut. Cf. the Late Latin name for it, nux Gallica, literally "Gaulish nut." Applied to the tree itself from 1600 (earlier walnut tree, c.1400).