'incompletely-documented' is a modern compound formed from the adverb 'incompletely' and the past participle 'documented'. 'in-' (negative prefix) comes from Latin 'in-' meaning 'not', 'complete' comes from Latin 'completus' via Old French, and 'document' comes from Latin 'documentum' (from 'docere', 'to teach' / 'to show').

'document' originated from Latin 'documentum' and entered English via Old French and Middle English as 'document'. 'complete' comes from Latin 'completus' (via Old French 'complet') and developed into English 'complete'. The adverb 'incompletely' is formed by prefixing 'in-' to 'completely'. The compound adjective 'incompletely-documented' is a relatively recent English formation used especially in technical and archival contexts.

Individually, 'document' originally referred to a written proof or lesson ('documentum'), and 'complete' meant 'filled up' or 'finished'; together as 'incompletely-documented' the phrase now specifically denotes that adequate documentation or records are missing or unfinished.