'non-aggressor' originates from the Latin prefix 'non' (from Latin 'non', meaning 'not') combined with the noun 'aggressor', ultimately from Latin 'aggredi' / past participle 'aggressor' meaning 'to approach, set upon, attack'.

'aggressor' entered English via Old French (e.g. 'agresseur') and Middle English forms, becoming English 'aggressor'; the productive negative prefix 'non-' (from Latin) was later attached in Modern English to form the compound 'non-aggressor'.

Initially it denotes simply 'not an attacker' (a negation of 'aggressor'); that negational sense has remained stable and 'non-aggressor' has not undergone major semantic change.