Etymology
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clock (n.1)

"machine to measure and indicate time mechanically" (since late 1940s also electronically), late 14c., clokke, originally "clock with bells," probably from Middle Dutch clocke (Dutch klok) "a clock," from Old North French cloque (Old French cloke, Modern French cloche "a bell"), from Medieval Latin clocca "bell," which probably is from Celtic (compare Old Irish clocc, Welsh cloch, Manx clagg "a bell") and spread by Irish missionaries (unless the Celtic words are from Latin). Ultimately of imitative origin.

Wherever it actually arose, it was prob. echoic, imitating the rattling made by the early handbells of sheet-iron and quadrilateral shape, rather than the ringing of the cast circular bells of later date. [OED]

Replaced Old English dægmæl, from dæg "day" + mæl "measure, mark" (see meal (n.1)). The Latin word was horologium (source of French horologe, Spanish reloj, Italian oriolo, orologio); the Greeks used a water-clock (klepsydra, literally "water thief;" see clepsydra).

The image of put (or set) the clock back "return to an earlier state or system" is from 1862. Round-the-clock (adj.) is from 1943, originally in reference to air raids. To have a face that would stop a clock "be very ugly" is from 1886. (Variations from c. 1890 include break a mirror, kill chickens.)

I remember I remember
That boarding house forlorn,
The little window where the smell
Of hash came in the morn.
I mind the broken looking-glass,
The mattress like a rock,
The servant-girl from County Clare,
Whose face would stop a clock.
[... etc.; The Insurance Journal, January 1886]
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clock (n.2)

"ornament pattern on a stocking," 1520s, probably identical with clock (n.1) in its older sense and meaning "bell-shaped ornament," though clock seems never to have been used for "bell" in English. Related: Clocked; clock-stocking.

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clock (v.)

"to time by the clock," 1883, from clock (n.1). The slang sense of "hit, sock" is 1941, originally Australian, probably from earlier slang clock (n.) "face" (1923). To clock in "register one's arrival by means of a mechanical device with a clock" is from 1914. Related: Clocked; clocking.

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clock-tower (n.)

"tower containing a clock," usually a large one with dials exposed on all four sides, 1757, from clock (n.1) + tower (n.). Older words for this were clocher (14c., from Old French), belfry.

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off (prep., adv.)

by c. 1200 as an emphatic form of Old English of (see of), employed in the adverbial use of that word. The prepositional meaning "away from" and the adjectival sense of "farther" were not firmly fixed in this variant until 17c., but once they were they left the original of with the transferred and weakened senses of the word. Meaning "not working" is from 1861.

Off the cuff "extemporaneously, without preparation" (1938) is from the notion of speaking from notes written in haste on one's shirt cuffs. In reference to clothing, off the rack (adj.) "not tailored, not made to individual requirements, ready-made" is by 1963, on the notion of buying it from the rack of a clothing store; off the record "not to be publicly disclosed" is from 1933; off the wall "crazy" is 1968, probably from the notion of a lunatic "bouncing off the walls" or else in reference to carom shots in squash, handball, etc.

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off (v.)

"to kill," 1930, from off (adv.). Earlier verbal senses were "to defer" (1640s), "to move off" (1882). Related: Offed.

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clock-watcher (n.)

"employee habitually prompt in leaving," 1887, from clock (n.1) + agent noun from watch (v.). Related: Clock-watching. Compare earlier tell-clock "idler" (c. 1600).

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clock-radio (n.)

"combined bedside radio and alarm clock which can be set to turn on the radio instead of sounding the alarm," 1946, from clock (n.1) + radio (n.).

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clock-maker (n.)

"one who makes clocks," mid-15c., from clock (n.1) + maker.

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