'antimilitaristically' originates from Modern English, formed by the negative prefix 'anti-' (from Greek 'antí', meaning 'against') attached to 'militaristic' (derived from 'military' with the adjectival suffix '-istic'); 'military' ultimately comes from Latin 'militaris' meaning 'of soldiers'.

'antimilitaristically' developed from Latin 'militaris' → Old French/Anglo-French terms for 'military' → Middle English 'military' → adjective 'militaristic' (formed by adding '-istic') → adding the prefix 'anti-' produced 'antimilitaristic', and finally the adverbial suffix '-ally' produced 'antimilitaristically'.

Initially terms from Latin ('militaris') meant 'relating to soldiers' or 'military'; with the addition of 'anti-' the meaning shifted to 'against military influence', and the adverb 'antimilitaristically' came to mean 'in a way that opposes militarism'.