What is the X in X-mas?
"blackbird," late 15c., from Old French merle "blackbird" (12c.), from Latin merulus "blackbird," from PIE *ams- "black, blackbird" (cf. Old English osle "blackbird;" see ouzel). The word owes its survival in modern times to its use by Scottish poets. The Latin word shows effects of rhotacism. It also is the source of Provençal and Spanish merla, Portuguese merlo, and Italian merla. Borrowed from French are Middle Dutch and German merle, Dutch meerle.