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.
<a href="https://www.etymonline.com/word/thro">Etymology of thro by etymonline</a>
Harper, D. (n.d.). Etymology of thro. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thro