'annexment' originates from Latin via Old French and Middle English, specifically from Latin 'annectere'/'annexus', where 'ad-' meant 'to/toward' and 'nectere' meant 'to bind', with the noun-forming suffix '-ment' (from Latin '-mentum') indicating the result or product.

'annexment' changed from Middle English/Old French forms such as 'annexement' and eventually became the modern English word 'annexment'.

Initially, it meant 'the act of joining or attaching'; over time it evolved into its current use meaning specifically 'the act of incorporating territory or adding something to a larger whole'.