What is the X in X-mas?
late 14c., from hind (adj.) + -most.
Thra. What, if a toy take 'em i' the heels now, and they run all away, and cry, 'The devil take the hindmost'? Dion. Then the same devil take the foremost too, and souse him for his breakfast! [Beaumont & Fletcher, "Philaster," Act V, Sc. 2, 1611]