What is the X in X-mas?
Old English behindan "behind, after," from bi "by" + hindan "from behind" (see hind (adj.)). The prepositional sense emerged in Old English. Euphemistic noun meaning "backside of a person" is from 1786. Phrase behind the times is from 1905. Behind the scenes (1711) is from the theater; figurative sense attested by 1779.
The buttocks; rump; ass: her broad, plain face, absence of waistline, and enormously broad behind (1830+)