'unjustifiableness' is formed by the prefix 'un-' (Old English/Proto-Germanic negative), the adjective 'justifiable' (via Old French 'justifier' from Latin), and the noun suffix '-ness' (Old English). The root ultimately traces to Latin 'justus' meaning 'just' and 'facere' meaning 'to make'.

'justify' comes from Old French 'justifier' and Latin 'justificare' (from 'justus' + 'facere'); Middle English produced 'justifiable' and with the negative prefix 'un-' and the suffix '-ness' the modern noun 'unjustifiableness' was formed.

Initially elements conveyed 'make just' (from Latin), and the assembled modern word has come to mean 'the state of being unable to be justified'; this negative nominal sense developed by productive use of the prefix 'un-' and suffix '-ness'.