Etymology
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chain (v.)
late 14c., "to bar with a chain; to put (someone) in chains," also "to link things together," from chain (n.). Related: Chained; chaining.
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chain (n.)

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." Meaning "a series of stores controlled by one owner or firm" is American English, 1846. Figurative use "that which binds or confines" is from c. 1600.

Chain-reaction is from 1916 in physics, 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.

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watch-chain (n.)
1739, from watch (n.) in the "timepiece" sense + chain (n.).
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chain-gang (n.)

"a number of slaves or convicts chained together outdoors doing labor or during transit," 1816, from chain (n.) + gang (n.). 

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chain-saw (n.)

also chain saw, chainsaw; 1818 as a surgical apparatus (for amputations) consisting of a chain, the links of which have serated edges; 1835 in the saw mill sense, "power-driven saw consisting of a chain with cutting points attached to the links;" from chain (n.) + saw (n.).

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unchain (v.)
1580s, from un- (2) "opposite of" + chain (v.). Figurative sense of "to liberate" is recorded from 1793. Related: Unchained; unchaining.
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keychain (n.)
also key-chain, 1849, from key (n.1) + chain (n.). Earlier in the same sense was key-ring (1889), key-band (mid-15c.).
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catenary (adj.)

"relating to a chain, like a chain or rope hanging freely from two fixed points," 1872, from Latin catenarius "relating to a chain," from catenanus "chained, fettered," from catena "chain, fetter, shackle" (see chain (n.)). As a noun in mathematics, "catenary curve," from 1788. Related: Catenarian.

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enchain (v.)
late 14c., "become linked together;" mid-15c., "to secure with a chain," from Old French enchainer, from Medieval Latin incatenare "enchain," from in (see in) + catenare, from catena "a chain" (see chain (n.)). Related: Enchained; enchaining.
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concatenation (n.)

c. 1600, "state of being linked together," from Late Latin concatenationem (nominative concatenatio) "a linking together," noun of action from past participle stem of concatenare "to link together," from com "with, together" (see con-) + catenare, from catena "a chain" (see chain (n.)). As "a series of things united like links in a chain" from 1726.

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