Thorough
Old English thuruh, alteration of thurh ‘through’. Original use was as an adverb and preposition, in senses of through . The adjective dates from the late 15th century, when it also had the sense ‘that goes or extends through something’, surviving in thoroughfare .
wiktionary
From Middle English thoruȝ, þoruȝ, from Old English þuruh, a byform of Old English þurh, whence comes English through. The adjective derives from the preposition and adverb. The word developed a syllabic form in cases where the word was fully stressed: when it was used as an adverb, adjective, or noun, and less commonly when used as a preposition.
etymonline
thorough (adj.)
c. 1300, adjectival use of Old English þuruh (adv.) "from end to end, from side to side," stressed variant of þurh (adv., prep.); see through. Related: thoroughly; thoroughness.
