'under-documented' originates from English, formed by the prefix 'under-' (from Old English 'under') and the past participle 'documented' (from 'document'), where 'under-' originally meant 'below' or 'less' and 'document' traces to Latin 'documentum'.

'document' changed from Latin 'documentum' (derived from 'docere' meaning 'to teach' or 'to show') into Old French 'document' and then Middle English 'document'; the past participle 'documented' came to mean 'recorded in writing', and combining it with 'under-' produced the modern descriptive phrase 'under-documented' meaning 'insufficiently recorded'.

Initially, components referred to 'below/less' ('under-') and 'lesson/proof' ('documentum'); over time 'document' shifted to mean a written record and the combined form evolved into the current sense 'insufficiently recorded or documented'.