thorough (adj.)

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.

*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."