'unnamable' originates from the negative prefix 'un-' (Old English), the root 'name' (Old English 'nama'), and the adjective-forming suffix '-able' (from Old French '-able' < Latin '-abilis'), where 'un-' meant 'not', 'nama' meant 'name', and '-able' meant 'capable of being'.

'name' changed from Old English 'nama' to Middle English 'name', while the suffix '-able' entered English via Old French from Latin '-abilis'; these elements combined with the negative prefix 'un-' to form the modern English adjective 'unnamable'.

Initially it could be understood simply as 'not capable of being given a name' (literal). Over time the word also acquired a figurative sense meaning 'unspeakable' or 'too extreme/terrible to be expressed', broadening from a literal to a more emotional/ethical use.