early 15c., "one of a series of rings or loops which form a chain; section of a cord," probably from Old Norse *hlenkr or a similar Scandinavian source (compare Old Norse hlekkr "link," in plural, "chain;" Old Swedish lænker "chain, link," Norwegian lenke "a link," Danish lænke "a chain," German Gelenk "articulation, a joint of the body; a link, ring"), from Proto-Germanic *khlink- (source also of German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn." Related to lank, flank, flinch.
The noun is not found in Old English, where it is represented by lank "the hip" ("turn of the body"), hlencan (plural) "armor." Meaning "a division of a sausage made in a continuous chain" is from mid-15c. Meaning "anything serving to connect one thing or part with another" is from 1540s. Sense of "means of telecommunication between two points" is from 1911. Missing link between man and apes dates to 1880.
"torch of tow, pitch, etc.," 1520s, of uncertain origin, possibly from Medieval Latin linchinus, from lichinus "wick," from Greek lykhnos "portable light, lamp," from PIE root *leuk- "light, brightness."
"to bind, fasten, couple, unite as if by links," late 14c., believed to be from link (n.1), though it is attested earlier. Intransitive sense "become connected, join in marriage" is from 1530s. Related: Linked; linking.