late 14c., in hunting, relai, "hounds placed along a line of chase" (to replace those that tire), from Old French relais "reserve pack of hounds or other animals; rest, stop, remission, delay" (13c.), from relaier "to exchange tired animals for fresh," literally "leave behind," from re- "back" (see re-) + laier "leave, let."
This is perhaps a variant of Old French laissier, (compare Old French relaisser "release"), from Latin laxare "slacken, undo" (see lax (adj.)). But Watkins has it from Frankish *laibjan, from a Proto-Germanic causative form of PIE root *leip- "to stick, adhere."
The etymological sense is "to leave (dogs) behind (in order to take fresh ones)." Of horses, 1650s. As "a squad of men to take a spell or turn of work at stated intervals," by 1808. As a type of foot-race, it is attested from 1898. The electromagnetic instrument is attested by name from 1860, originally in telegraphy.
c. 1400, relaien, "to set a pack of (fresh) hounds after a quarry;" also "change horses, take a fresh horse," from Old French relaiier, from relai (see relay (n.)). The word seems to have faded out by 19c. but was re-formed in electromagnetics from the noun, in a transitive sense of "pass on or retransmit," originally of telephone signals (1878), later in a transferred sense of "pass on information" (by 1956). Related: Relayed; relaying.
c. 1300, "a movement, a beginning," from shift (v.); by mid-15c. as "an attempt, expedient, or means." This is the word in make shift "make efforts" (mid-15c.; see makeshift). The specific sense of "means to an end" is from 1520s, hence "a device, a trick." The sense of "change, alteration" in character, place, position, etc., is from 1560s.
The meaning "mechanism for changing gear in a motor vehicle" is recorded from 1914. Typewriter shift key is so called by 1893; its shift-lock is so called from 1899.
The meaning "period of working time" (originally in a mine) is attested from 1809, perhaps from or influenced by an older sense "relay of horses" (1708); perhaps also influenced by a North Sea Germanic cognate word (such as North Frisian skeft "division, stratum," skaft "one of successive parties of workmen"). Similar double senses of "division" and "relay of workers" is in Swedish skift, German schicht.
mid-14c., relien, "to gather, assemble" an army, followers, a host, etc. (transitive and intransitive), from Old French relier "assemble, put together; fasten, fasten again, attach, rally, oblige," from Latin religare "fasten, bind fast," from re-, here perhaps an intensive prefix (see re-), + ligare "to bind" (from PIE root *leig- "to tie, bind").
The older sense now are obsolete. The meaning "depend on with full trust and confidence, attach one's faith to" a person or thing is from 1570s, perhaps via the notion of "rally to, fall back on." Typically used with on, perhaps by influence of unrelated lie (v.2) "rest horizontally." Related: Relied; relying.
The verb rely, in the orig. sense 'fasten, fix, attach,' came to be used with a special reference to attaching one's faith or oneself to a person or thing (cf. 'to pin one's faith to a thing,' 'a man to tie to,' colloquial phrases containing the same figure); in this use it became, by omission of the object, in transitive, and, losing thus its etymological associations (the other use, 'bring together again, rally,' having also become obsolete), was sometimes regarded, and has been by some etymologists actually explained, as a barbarous compound of re- + E. lie (1) rest, .... But the pret. would then have been *relay, pp. *relain. [Century Dictionary]