thorough (adj.)
Middle English thoro, thorow, “perfect, complete,” mid-13c., a two-syllable stressed form of thurgh “passing or cutting all the way through,” which is an adverb (represented by modern through, “from end to end, from side to side”) used as an adjective. The notion in thorough is “going all the way through.“
Þurh-, thurgh- was an active word-forming element and prefix in Old English and Middle English, often in making transitive verbs of motion (thurghcomen, thurghgon, thurghfallen, thurghserchen, and compare thoroughfare) or intensive adjectives (thurgh-fin, thurgh-hot, thurgh-stif, thurgh-wet, and compare thoroughgoing). It also often translates Latin per-.
The stressed form of through began to develop in the adverb in late Old English. The stress and spelling change seems to not directly track with the sense shift. For the form, compare borough from Old English burh, furrow from furh. Related: Thoroughly; thoroughness.
The Old English adverb is attested as þurh, þurg, þuruh, þorh, þorch, þerh. It became through, the common modern form, by transposition.
also from mid-13c.