What is the X in X-mas?
Old English sceað, scæð, from Proto-Germanic *skaithiz (cf. Old Saxon scethia, Old Norse skeiðir (plural), Old Frisian skethe, Middle Dutch schede, Dutch schede, Old High German skaida, German scheide "a sheath, scabbard"), according to OED, possibly from root *skei- "divide, split" (see shed (v.)) on notion of a split stick with the sword blade inserted. Meaning "condom" is recorded from 1861; sense of "close-fitting dress or skirt" is attested from 1904.
n. pl. sheaths (shēðz, shēths)
An enveloping tubular structure, such as the tissue that encloses a muscle or nerve fiber.