Etymology
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Words related to thorough

through (prep., adv.)

late 14c., metathesis of Old English þurh, from Proto-Germanic *thurx (source also of Old Saxon thuru, Old Frisian thruch, Middle Dutch dore, Dutch door, Old High German thuruh, German durch, Gothic þairh "through"), from PIE root *tere- (2) "to cross over, pass through, overcome." Not clearly differentiated from thorough until early Modern English. Spelling thro was common 15c.-18c. Reformed spelling thru (1839) is mainly American English.

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*tere- (2)

*terə- Proto-Indo-European root meaning "cross over, pass through, overcome."

It forms all or part of: avatar; caravanserai; nectar; nectarine; nostril; seraglio; thrill; thorough; through; tranche; trans-; transient; transom; trench; truculent; truncate; trunk.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tirah, Avestan taro "through, beyond;" Latin trans "beyond;" Old Irish tre, Welsh tra "through;" Old English þurh "through."

thoro 
abbreviated spelling of thorough.
thoroughbred (adj.)
1701, of persons, "thoroughly accomplished," from thorough + past tense of breed. In reference to horses, "of pure breed or stock," from 1796; the noun is first recorded 1842.
thoroughfare (n.)
late 14c., "passage or way through," from thorough (before it had differentiated from through) + fare (n.).