'unjustifiability' originates from English, formed by the negative prefix 'un-' (not) added to 'justifiability'. 'Justifiability' is ultimately derived from Latin through Old French: Latin 'justificare' (to make just) > Old French 'justifier' > English 'justify' + nominalizing suffixes.

The element 'justus' (Latin, 'just') combined with 'facere' ('to make') produced 'justificare' in Late Latin; this became Old French 'justifier', then Middle/Modern English 'justify' and 'justification'. From 'justification' the noun form 'justifiability' developed in English, and the negative form 'unjustifiability' was formed by prefixing 'un-'.

Originally tied to the idea of 'making just' or 'showing to be just', the family of words came to mean 'capable of being shown to be reasonable or defensible'; with the prefix 'un-' the meaning became 'not capable of being justified', which is the current sense.