watch-chain (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]

c. 1300, "connected series of links of metal or other material," from Old French chaeine "chain" (12c., Modern French chane), from Latin catena "chain" (source also of Spanish cadena, Italian catena), which is of unknown origin, perhaps from a PIE root *kat- "to twist, twine" (source also of Latin cassis "hunting net, snare").

As a type of ornament worn about the neck, from late 14c. As a linear measure ("a chain's length") from 1660s. From 1590s as "any series of things linked together." The meaning "series of stores controlled by one owner or firm" is American English, 1846. The figurative use "that which binds or confines" is from c. 1600.

Chain-reaction is from 1916 in physics; the specific nuclear physics sense is from 1938. Chain-mail armor is from 1795, from mail (n.2). Before that, mail alone sufficed. Chain letter is recorded from 1892; at first usually to raise money; decried from the start as a nuisance.

Nine out of every ten givers are reluctant and unwilling, and are coerced into giving through the awful fear of "breaking the chain," so that the spirit of charity is woefully absent. [St. Nicholas magazine, vol. xxvi, April 1899] 

Chain of command is from 1915. Chain-lightning, visible as jagged or broken lines, is from 1834. Chain-smoker, one who smokes one after another, lighting the next from the stump of the last, is attested from 1885, originally of Bismarck (who smoked cigars), thus probably a loan-translation of German Kettenraucher. Chain-smoking (n.) is from 1895.