Etymology
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tachycardia (n.)

"rapid heartbeat," 1868, Modern Latin, coined 1867 by German-born physician Hermann Lebert (1813-1878) from tachy- "swift" + Latinized form of Greek kardia "heart," from PIE root *kerd- "heart."

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tachygraphy (n.)

"shorthand, stenography," 1640s, from Latinized form of Greek takhygraphia, from takhys "swift" (see tachy-) + -graphia (see -graphy). Related: Tachygraphic; tachygrapher "stenographer" (especially among the ancients; see Tironian).

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tachymeter (n.)

surveying instrument, 1836, from tachy- "swift" + -meter. Related: Tachymetry.

M. GAETANO CAÏRO has invented an instrument, to which he has given the name of Tachymeter (rapid measurer). Its object is to give the area of plane surfaces bounded by any outline whatever, without the necessity of any arithmetical operation. [Magazine of Popular Science and Journal of the Useful Arts, vol. ii, 1836]
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tachyon (n.)

1967, hypothetical faster-than-light particle, from tachy- "swift" + -on.

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tachypnea (n.)

"hysterical rapid breathing," 1896, from tachy- "swift" + -pnea, from pnein "to breathe" (see pneuma). Related: Tachypneic.

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tacit (adj.)

c. 1600, "silent, unspoken," from French tacite and directly from Latin tacitus "that is passed over in silence, done without words, assumed as a matter of course, silent," past participle of tacere "be silent, not speak," from suffixed form of PIE root *tak- "to be silent" (source also of Gothic þahan, Old Norse þegja "to be silent," Old Norse þagna "to grow dumb," Old Saxon thagian, Old High German dagen "to be silent"). The musical instruction tacet is the 3rd person present singular of the Latin verb. Related: Tacitly.

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taciturn (adj.)

"habitually silent," 1771, back-formation from taciturnity, or from French taciturne (15c.), from Latin taciturnus "not talkative, noiseless."

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taciturnity (n.)

mid-15c., from Old French taciturnité, from Latin taciturnitatem (nominative taciturnitas) "a being or keeping silent," from taciturnus "disposed to be silent," from tacitus "silent" (see tacit).

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tack (n.1)

"clasp, hook, fastener," also "a nail" of some kind, c. 1400, from Old North French taque "nail, pin, peg" (Old French tache, 12c., "nail, spike, tack; pin brooch"), probably from a Germanic source (compare Middle Dutch tacke "twig, spike," Frisian tak "a tine, prong, twig, branch," Low German takk "tine, pointed thing," German Zacken "sharp point, tooth, prong"), from Proto-Germanic *tag-. Meaning "small, sharp nail with a flat head" is attested from mid-15c. The meaning "rope to hold the corner of a sail in place" is first recorded late 15c.

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tack (v.1)

late 14c., "to attach" with a nail, etc., from tack (n.1). Meaning "to attach as a supplement" (with suggestion of hasty or arbitrary proceeding) is from 1680s. Related: Tacked; tacking.

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