'unilateralness' is formed from the adjective 'unilateral' plus the noun-forming suffix '-ness'; 'unilateral' itself comes from the Latin prefix 'uni-' meaning 'one' and Latin root 'latus' meaning 'side'.

'unilateral' developed in modern English from Medieval Latin/French formations combining 'uni-' and 'lateralis' (from Latin 'latus'); the English suffix '-ness' (from Old English '-nes(s)') was later added to create the abstract noun 'unilateralness'.

Initially related to the literal elements 'one' and 'side' (i.e., 'one-sided'), the combined form has retained that core meaning and came to denote both the state of being one-sided and, by extension, instances of one-sided action.