'anti-annexationism' originates from Greek and Latin elements: the prefix 'anti-' (from Greek 'anti', meaning 'against'), combined with 'annexation' (from Latin 'annexare'/'annexus', meaning 'to join or bind to'), and the English nominal suffix '-ism' (from Greek/Latin '-ismos'/'-ismus', forming doctrines or systems).

'anti-annexationism' was formed by attaching the suffix '-ism' to the compound 'anti-annexation' (itself built from 'anti-' + 'annexation'). 'Annex' and related forms entered English via Latin (annexare/annexus) often through Old French and Middle English, and the modern English compound developed in political discourse to name the opposing doctrine.

Initially the components simply meant 'against' + 'joining/binding' (i.e., opposition to joining). Over time the compound came to specifically denote an organized doctrine, movement, or sentiment opposing territorial annexation, which is its current specialized political meaning.