'non-annexable' is formed from the negative prefix 'non-' (from Latin 'non' meaning 'not') added to 'annexable', which itself comes from 'annex' + the adjectival suffix '-able'.

'annex' derives from Latin 'annexus' (past participle of 'annectere' or related forms meaning 'to bind to'), passed into Old French as 'anexer'/'anexer' and into Middle English as 'annex'; the suffix '-able' comes via Old French from Latin '-abilis'; these combined to form 'annexable', to which the prefix 'non-' was later attached to create 'non-annexable'.

Initially related to the idea 'to bind or join' (Latin roots), the word evolved to mean 'to attach territorially or incorporate'; 'non-annexable' therefore came to mean 'not capable of being incorporated or annexed'.