'annexal' originates from Latin, specifically from the past participle 'annexus' (from 'annectere'), where the prefix 'ad-' (assimilated to 'an-') meant 'to' and 'nectere' meant 'to bind'.

'annexus' passed into Medieval/Modern Latin and Old French forms (e.g. 'annexer') and then into English as 'annex'; the adjective form 'annexal' was formed in English by adding the suffix '-al' to 'annex'.

Initially it conveyed the sense 'joined or bound to' (attached); over time it retained this core meaning and came to be used both for buildings/parts and, in medicine, for anatomical appendages.