'antiexpansionism' is formed from the prefix 'anti-' (against) + 'expansionism' (the doctrine or policy of expansion).

'expansionism' comes from 'expansion' (from Latin 'expansio', from 'expandere' = ex- 'out' + pandere 'to spread') which passed into English via Old French/Medieval Latin; the adjective/noun-forming suffix '-ism' was added in modern English, and the productive prefix 'anti-' (from Greek 'anti-' via Latin/Old French) was attached to form 'antiexpansionism' in modern political discourse.

The components originally denoted 'against' and 'spreading/outward movement'; combined, the term has meant 'opposition to policies of expansion' since its formation and retains that specific political sense.