stopwatch (n.)

Old English -stoppian (in forstoppian "to stop up, stifle"), a general West Germanic word, cognate with Old Saxon stuppon, West Frisian stopje, Middle Low German stoppen, Old High German stopfon, German stopfen "to plug, stop up," Old Low Frankish (be)stuppon "to stop (the ears)."

These words are said by many sources to be a Germanic borrowing of Vulgar Latin *stuppare "to stop or stuff with tow or oakum" (source of Italian stoppare, French étouper "to stop with tow"), from Latin stuppa "coarse part of flax, tow." In support of this theory, it is said that plugs made of tow were used from ancient times in Rhine valley. Century Dictionary says this "suits phonetically," but "is on grounds of meaning somewhat doubtful." Barnhart, for one, proposes the whole Germanic group might be native, from a base *stoppon.

Sense of "bring or come to a halt, discontinue" (mid-15c.) is from notion of preventing a flow by blocking a hole, and the word's development in this sense is unique to English, though it since has been widely adopted in other languages; perhaps influenced by Latin stupere "be stunned, be stupefied." Intransitive meaning "check oneself" is from 1680s. Meaning "make a halt or stay, tarry" is from 1711. Stop-light is from 1922; stop-sign is from 1918. Stop-motion is from 1851, originally of looms. Related: Stopped; stopping.

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]