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c.1300, metathesis of Old English þurh, from West Germanic *thurkh (cf. Old Saxon thuru, Old Frisian thruch, Middle Dutch dore, Dutch door, Old High German thuruh, German durch, Gothic þairh "through"), from PIE root *tere- "to cross over, pass through, overcome" (cf. 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 (1917) is mainly American English.