late 14c., metathesis of Old English þurh, from Proto-Germanic *thurkh (cognates: 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" (cognates: Sanskrit tirah, Avestan taro "through, beyond," Latin trans "beyond," Old Irish tre, Welsh tra "through"). Not clearly differentiated from thorough until early Modern English. Spelling thro was common 15c.-18c. Reformed spelling thru (1839) is mainly American English.