'ill-documented' is formed in English from the prefix 'ill-' and the past-participial adjective 'documented'. The prefix 'ill-' goes back to Old English 'ill' meaning 'bad' or 'not well', while 'documented' derives from Latin 'documentum' (from 'docere') where 'docere' meant 'to teach' or 'to show (as evidence)'.

'documentum' (Latin) passed into Old French as 'document' and into Middle English as 'document'; the past participle/adjectival form 'documented' developed in Modern English. The negative/adversative prefix 'ill-' has been used in English since Old/Middle English to form compounds (e.g. ill‑fated), and was combined with 'documented' to yield 'ill-documented'.

Originally the elements meant 'badly' (ill-) + 'shown/proved by records' ('documented'); together they have long meant 'badly or insufficiently supported by documentation' — a meaning that has remained stable.