'nonapportionability' originates from English, formed by the prefix 'non-' (not) and 'apportionability' derived from 'apportion' + '-ability'. 'apportion' ultimately comes from Old French and Latin roots related to 'portion' or 'part'.

'apportion' came into English from Old French apportioner (from Late Latin portionare) meaning 'to divide into portions'; the adjective and noun forms were built in Middle and Modern English with the suffixes '-able' and '-ability'; the modern compound 'non-' + 'apportionability' is a more recent English formation used in legal and formal contexts.

Originally tied to the action 'to divide into parts' (to give portions), the element developed into nouns and adjectives denoting the capacity to be divided; 'nonapportionability' therefore evolved to mean 'the inability to be apportioned' in current usage.