watchyoudaoicibaDictYouDict[watch 词源字典]
watch: [OE] Ultimately, watch and wake are the same word. The two verbs share a common ancestor (prehistoric Germanic *wakōjan), and to begin with watch was used for ‘be awake’ (‘He sleepeth on the day and watcheth all the night’, John Lydgate, 1430). The notion of being ‘alert and vigilant’, of being ‘on the look-out’, is implicit in that of being ‘awake’ (indeed, vigil and vigilant are members of the same word family), but watch did not develop fully into ‘observe, look at closely’ until the 14th century.

The sort of watch that tells the time is probably so called not because you look at it to see what the time is, but because originally it woke you up. The earliest records of the noun’s application to a timepiece (in the 15th century) refer to an ‘alarm clock’; it was not used for what we would today recognize as a ‘watch’ until the end of the 16th century.
=> vegetable, vigil, vigour, waft, wait, wake[watch etymology, watch origin, 英语词源]
watch (v.)youdaoicibaDictYouDict
Old English wæccan "keep watch, be awake," from Proto-Germanic *wakjan, from PIE *weg- (2) "to be strong, 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.
watch (n.)youdaoicibaDictYouDict
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 (see watch (v.)). 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.
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 frumberdligges; hanecrau þe bilimpeð þowuene men; morgewile to alde men. [Trinity Homilies, c. 1200]
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.).