'unassessability' originates from Modern English, specifically formed from the negative prefix 'un-' + the adjective 'assessable' (itself 'assess' + '-able') + the noun-forming suffix '-ity'.

'assess' comes into English via Old French 'assesser' / Medieval Latin 'assessare' from Latin components 'ad-' (toward) + 'sedere' (to sit); 'assessable' developed from 'assess' + '-able', and the modern noun 'unassessability' was formed by adding 'un-' and '-ity' to express the state of not being assessable.

Initially the Latin root was about 'sitting (by)' (related to determining or deciding by sitting), which evolved into the legal/administrative sense of 'fixing a value' or 'evaluating'; over time this developed into the modern sense of 'to evaluate or determine (a value)', and 'unassessability' now means 'the state of being not able to be evaluated'.