The word 'thorough' originates from the Old English adjective and adverb 'thurh', a variant of 'thuruh', meaning "through." It is related to the Old English preposition 'þurh', from which the modern word "through" also derives. This Old English term can be traced back to the Proto-Germanic 'thurkh', and further to the Proto-Indo-European root 'tr̥h₂-', meaning "to pass through" or "to overcome." Over time, the adverbial sense of "through and through" or "completely" evolved into the modern adjective 'thorough', embodying the idea of something done from one end to the other, leaving no part unexamined. The spelling was influenced in Middle English by an association with words like 'borough' and 'burrow', solidifying its distinct form from 'through'.

The word is a simplex form. Its spelling and form have been shaped by a long and complex phonetic evolution from Old English, making a segmentation into meaningful modern English substrings (like 'thor' + 'ough') linguistically invalid and misleading for learners.