英语词源字典 - 在线词源网站

down (adv.)[down 词源字典]: late Old English shortened form of Old English ofdune "downwards," from dune "from the hill," dative of dun "hill" (see down (n.2)). A sense development peculiar to English.

Used as a preposition since c. 1500. Sense of "depressed mentally" is attested from c. 1600. Slang sense of "aware, wide awake" is attested from 1812. Computer crash sense is from 1965. As a preposition from late 14c.; as an adjective from 1560s. Down-and-out is from 1889, American English, from situation of a beaten prizefighter. Down home (adj.) is 1931, American English; down the hatch as a toast is from 1931; down to the wire is 1901, from horse-racing. Down time is from 1952. Down under "Australia and New Zealand" attested from 1886; Down East "Maine" is from 1825; Down South "in the Southern states of the U.S." is attested by 1834.[down etymology, down origin, 英语词源]
layoff (n.): also lay-off, lay off; 1889, "rest, respite;" from lay (v.) + off. Via seasonal labor with periodic down time, it came to have a sense of "temporary release from employment," and by 1960s was being used somewhat euphemistically for permanent releases of masses of workers by employers. The verbal phrase lay off is attested from 1868 as "dismiss" (an employee); meaning "stop disturbing" is from 1908.