word-forming element of Greek origin meaning "speed," often short for tachometer, ultimately from Latinized form of Greek takhos "speed, swiftness, fleetness, velocity," which Beekes calls a derivative of takhys "swift," a word of unknown origin.

also tacheometer, "speed-measuring instrument for a machine or engine," 1810, coined by its inventor, Bryan Donkin (1768-1855), from tacho- "speed" + -meter. Related: Tachometry.

"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").

"shorthand, stenography, the art of writing in abbreviations," 1640s, from Latinized form of Greek takhygraphia, from takhys "swift" (see tachy-) + -graphia (see -graphy). Related: Tachygraphic; tachygrapher (1849) "stenographer, short-hand taker" (modern or ancient; compare Tironian); tachygraphist (1825).

surveying instrument for rapidly locating position, 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]

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

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

c. 1600, "unspoken, noiseless, wordless; saying nothing, silent," by 1630s as "silently indicated or implied (in tacit approving), 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." This is reconstructed (Watkins) to be from a 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." Related: Tacitly.

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

"habitually silent, reserved in speech," 1650s, back-formation from taciturnity, or from French taciturne (15c.), from Latin taciturnus "not talkative; noiseless."