'annexe' originates from French, specifically the verb 'annexer', where 'an-' is a form of Latin 'ad-' meaning 'to' and 'nex' relates to Latin 'nectere' meaning 'to bind'.

'annexer' (Old French) passed into Middle English as 'annexen' or 'annex', and later developed the noun spelling 'annexe' in British English while 'annex' became common in American English.

Initially it meant 'to bind or attach (something)', and over time it evolved into the current meanings of 'attach/add on' (for buildings or documents) and 'incorporate/annex' (for territory).