'annexational' ultimately originates from Latin, specifically the word 'annexus', where the prefix 'ad-' (in the form assimilated to 'an-') meant 'to' and the root 'nectere' (or 'nect-') meant 'to bind or tie'.

'annexational' developed from Medieval/Modern Latin forms such as 'annexare' and Old French/Medieval borrowings (e.g. Old French 'anexer'/'annexer'), through Middle English 'annex'/'annexion' and then English 'annexation' and the adjective-forming suffix -al to yield 'annexational'.

Initially it carried the sense of 'joined' or 'bound to' (literally 'tied to'); over time it evolved to the political/administrative sense 'to add or incorporate (territory)', and 'annexational' now means 'relating to such annexation'.