by 1839, altered spelling of through; at first often in representations of dialect (Scottish, Yankee), by 1880s in standard use as a simplified spelling.
Entries linking to thru
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.