preposition & adverb
Old English thurh (preposition and adverb), of Germanic origin; related to Dutch door and German durch. The spelling change to thr- appears circa1300, becoming standard from Caxton onwards
Do not confuse thorough with through. Thorough means 'done with great care and completeness' (officers made a thorough examination of the wreckage), whereas through means 'in one side and out the other' (she stepped boldly through the door).