"in or farther toward the back part (of a ship)," as opposed to forward, 1590s, from Middle English on baft (late 13c.) "back, behind, to the rear," from Old English on bæftan. For first element, see a- (1). The second component is itself a compound of be "by" (see by) and æftan "aft" (see aft). The word has been saved by the sailors (the stern being the "after" part of a vessel), the lubbers having left it in Middle English.
Old English behindan "at the back of, after," from bi "by" (see by) + hindan "from behind" (see hind (adj.)). The prepositional sense emerged in Old English. Figurative sense "not so far advanced, not on equality with" is from c. 1200. Euphemistic noun meaning "backside of a person" is from 1786. To do something behind (someone's) back "clandestinely" is from late 14c. Phrase behind the times is by 1826. Behind the scenes (1711) is from the theater; figurative sense attested by 1779.
Old English butan, buton "unless; with the exception of; without, outside," from West Germanic *be-utan, a compound of *be- "by" (see by) + *utana "out, outside; from without," from ut "out" (see out (adv.)). Not used as a conjunction until late Old English, "on the contrary." Senses attested in early Middle English include "however, yet; no more than." As an introductory expression, early 13c. As a noun, "an objection, an exception" from late 14c.
Som man preiseth his neighebore by a wikked entente, foralwey he maketh a 'but' at the laste ende, that is digne of moore blame than worth is al the preisynge. [Chaucer, "Parson's Tale"]