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Old English wæccan "keep watch, be awake," from Proto-Germanic *wakojan; essentially the same word as Old English wacian "be or remain awake" (see wake (v.)); perhaps a Northumbrian form. 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.
Old English wæcce "a watching," from wæccan (see watch (v.)). Sense of "sentinel" is recorded from c.1300; that of "person or group officially patroling a town (especially at night) to keep order, etc." is first recorded 1530s. Meaning "period of time in which a division of a ship's crew remains on deck" is from 1580s. Sense of "period into which a night was divided in ancient times" translates 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]The meaning "small timepiece" is from 1580s, developing from that of "a clock to wake up sleepers" (mid-15c.).
the periods into which the time between sunset and sunrise was divided. They are so called because watchmen relieved each other at each of these periods. There are frequent references in Scripture to the duties of watchmen who were appointed to give notice of the approach of an enemy (2 Sam. 18:24-27; 2 Kings 9:17-20; Isa. 21:5-9). They were sometimes placed for this purpose on watch-towers (2 Kings 17:9; 18:8). Ministers or teachers are also spoken of under this title (Jer. 6:17; Ezek. 33:2-9; Heb. 13:17). The watches of the night were originally three in number, (1) "the beginning of the watches" (Lam. 2:19); (2) "the middle watch" (Judg. 7:19); and (3) "the morning watch" (Ex. 14:24; 1 Sam. 11:11), which extended from two o'clock to sunrise. But in the New Testament we read of four watches, a division probably introduced by the Romans (Matt. 14:25; Mark 6:48; Luke 12:38). (See DAY.)