'unilateralism' originates from the adjective 'unilateral' plus the suffix '-ism'. 'Unilateral' itself derives from Latin elements 'uni-' meaning 'one' and 'latus' meaning 'side'; the suffix '-ism' comes from Greek '-ismos' via Latin and French, marking doctrines or practices.

'unilateral' developed from Latin 'unilateralis' (formed from 'uni-' + 'lateralis'), passed through Romance-language forms (e.g. French 'unilatéral'), and entered English as 'unilateral'; English formed the noun 'unilateralism' by adding '-ism' to describe the doctrine or practice.

Initially it described something 'pertaining to one side' (literal sense); over time it came to mean the policy or doctrine of acting alone or one-sidedly (especially in politics and international relations).