Etymology
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watch (n.)

Old English wæcce "a watching, state of being or remaining awake, wakefulness;" also "act or practice of refraining from sleep for devotional or penitential purposes;" from wæccan "keep watch, be awake," from Proto-Germanic *wakjan, from PIE root *weg- "to be strong, be lively."

From c. 1200 as "one of the periods into which the night is divided," in reference to ancient times translating Latin vigilia, Greek phylake, Hebrew ashmoreth. From mid-13c. as "a shift of guard duty; an assignment as municipal watchman;" late 13c. as "person or group obligated to patrol a town (especially at night) to keep order, etc."

Also in Middle English, "the practice of remaining awake at night for purposes of debauchery and dissipation;" hence wacches of wodnesse "late-night revels and debauchery." The alliterative combination watch and ward preserves the old distinction of watch for night-time municipal patrols and ward for guarding by day; in combination, they meant "continuous vigilance."

Military sense of "military guard, sentinel" is from late 14c. General sense of "careful observation, watchfulness, vigilance" is from late 14c.; to keep watch is from late 14c. Meaning "period of time in which a division of a ship's crew remains on deck" is from 1580s. The meaning "small timepiece" is from 1580s, developing from that of "a clock to wake up sleepers" (mid-15c.).

The Hebrews divided the night into three watches, the Greeks usually into four (sometimes five), the Romans (followed by the Jews in New Testament times) into four. [OED]
On þis niht beð fowuer niht wecches: Biforen euen þe bilimpeð to children; Mid-niht ðe bilimpeð to frumberdlinges; hanecrau þe bilimpeð þowuene men; morgewile to alde men. [Trinity Homilies, c. 1200]
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watch (v.)
Old English wæccan "keep watch, be awake," from Proto-Germanic *wakjan, from PIE root *weg- "to be strong, be lively." Essentially the same word as Old English wacian "be or remain awake" (see wake (v.)); perhaps a Northumbrian form of it. Meaning "be vigilant" is from c. 1200. That of "to guard (someone or some place), stand guard" is late 14c. Sense of "to observe, keep under observance" is mid-15c. Related: Watched; watching.
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watch-chain (n.)
1739, from watch (n.) in the "timepiece" sense + chain (n.).
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watch-work (n.)
1660s, from watch (n.) in the "timepiece" sense + work (n.).
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night-watch (n.)
"guard kept during the night," late Old English; see night + watch (n.).
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death-watch (n.)

"a vigil beside a dying person," 1865, from death + watch (n.) "a watching." The death-watch beetle (1660s) inhabits houses, makes a ticking noise like a pocket-watch, and was superstitiously supposed to portend death.

FEW ears have escaped the noise of the death-watch, that is, the little clicking sound heard often in many rooms, somewhat resembling that of a watch; and this is conceived to be of an evil omen or prediction of some person's death: wherein notwithstanding there is nothing of rational presage or just cause of terror unto melancholy and meticulous heads. For this noise is made by a little sheathwinged grey insect, found often in wainscot benches and wood-work in the summer. [Browne, "Vulgar Errors"] 
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watchmaker (n.)
1620s, from watch (n.) in the "timepiece" sense + maker.
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watchtower (n.)
also watch-tower, 1540s, from watch (v.) + tower (n.).
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watcher (n.)
late 14c. (early 13c. as a surname), agent noun from watch (v.).
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watchdog (n.)
also watch-dog, c. 1600, from watch (v.) + dog (n.). Figurative sense is attested by 1845.
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