'underdocumented' originates from English, formed by the prefix 'under-' + the past participle 'documented' (from 'document'). 'under-' comes from Old English 'under' meaning 'below' or 'less than', and 'document' derives from Latin 'documentum'.

'document' came into English via Latin 'documentum' (from 'docēre' 'to teach') and Old French 'document', then Middle English 'document'. The adjective 'underdocumented' is a modern English compound created by combining 'under-' with 'documented' to indicate inadequate documentation.

Originally 'documentum' meant 'a lesson, example, or proof' and 'document' gradually came to mean a written record. 'Under-' originally meant 'below' or 'beneath' and developed a sense of 'insufficient' in compounds like 'underdone' or 'undervalued'; together they now convey 'not sufficiently recorded'.