'thoroughly-documented' originates from English, specifically a compound of 'thoroughly' (from Old English 'thurh' meaning 'through' + the adverbial suffix '-ly') and 'documented' (from Latin 'documentum' meaning 'lesson; proof', via Old French and Middle English 'document' + the English participial suffix '-ed').

'documentum' in Latin passed into Old French as 'document', then into Middle English as 'document', from which the verb/adjective 'document(ed)' developed in Modern English. In parallel, Old English 'thurh' developed into 'thorough' and then the adverb 'thoroughly'. These elements combined in Modern English to form the compound adjective 'thoroughly-documented'.

Originally, 'documentum' denoted a 'lesson' or 'example', later narrowing to 'proof' and 'written record'. 'Documented' came to mean 'recorded in documents'. With 'thoroughly', the compound emphasizes the completeness and detail of the documentation in modern usage.