Etymology
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valve (n.)
late 14c., "one of the halves of a folding door," from Latin valva (plural valvae) "section of a folding or revolving door," literally "that which turns," related to volvere "to roll," from PIE root *wel- (3) "to turn, revolve." Sense extended 1610s to "membranous fold regulating flow of bodily fluids;" 1650s to "mechanical device that works like an anatomical valve;" and 1660s in zoology to "halves of a hinged shell." Related: Valved.
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univalve 
of mollusks and shells, 1660s (noun and adjective), from uni- + valve.
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bivalve (adj.)
1660s in reference to mollusks with hinged double shells; 1670s in reference to shutters or doors having two folding parts; from bi- + valve. The noun is 1680s in the mollusk sense.
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mitral (adj.)

c. 1600, "resembling a mitre, of or pertaining to a mitre," from French mitral, from Modern Latin mitralis, from Latin mitra (see mitre). The mitral valve of the heart is so called from 1705, from Modern Latin mitrales valvulae.

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choke (n.)

1560s, "quinsy," from choke (v.). Meaning "action of choking" is from 1839. Meaning "valve which controls air to a carburetor" first recorded 1926; earlier it meant "constriction in the bore of a gun" (1875).

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reflux (n.)

early 15c., "a flowing back" (of the sea, etc.), "ebb tide," also figurative of instability, from Medieval Latin refluxus, from Latin re- "back, again" (see re-) + fluxus "a flowing," from fluere "to flow" (see fluent). Digestive sense is recorded from 1937; reflux-valve is attested by 1853.

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throttle (n.)
1540s, "throat;" it appears to be an independent formation from throat, perhaps a diminutive form, not derived directly from the verb. The mechanical sense is first recorded 1872, short for throttle-valve (1824). Full-throttle (allowing maximum speed) is from 1848 in reference to steam engines.
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bypass (n.)
also by-pass, 1848, "small pipe passing around a valve in a gasworks" (for a pilot light, etc.), from the verbal phrase; see by + pass (v.). First used 1922 for "road for the relief of congestion;" figurative sense is from 1928. The heart operation was first so called 1957.
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spigot (n.)
late 14c., "plug used to stop the hole of a cask," according to Barnhart probably from Old French *espigot (compare Gascony dialect espigot "core of a fruit, small ear of grain"), diminutive of Old Provençal espiga "ear of grain," from Latin spica "ear of grain" (see spike (n.2)). Meaning "valve for controlling the flow of a liquid" is from 1520s; the connecting notion is "that which controls or restrains."
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cock (n.2)

in various mechanical senses, such as "turn-valve of a faucet" (early 15c.), of uncertain connection with cock (n.1). Perhaps all are based on real or fancied resemblances not now obvious; German has hahn "cock" in many of the same senses.

The cock of a firearm, which when released by the action of the trigger discharges the piece, is from 1560s. Hence "position into which the hammer is brought by being pulled back to the catch" (1745). For half-cocked, see cock (v.).

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