thorough: [OE] Thorough is ultimately the same word as through. Both go back to Old English thurh ‘through’. In its single-syllable form it has stuck to its original role as a preposition and adverb, but in the case of thorough this has now virtually died out, leaving only the adjective, which evolved in the 13th century from the notion of going ‘through’ something so as to affect every part. => thrill, through
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.
