'horribleness' is formed in English by adding the noun-forming suffix '-ness' to 'horrible' (from Latin 'horribilis'), creating the meaning 'the state or quality of being horrible'.

'horrible' entered English via Old French and Middle English from Latin 'horribilis' (from the verb 'horrēre', 'to bristle, shudder'); the modern English noun 'horribleness' developed by combining 'horrible' + '-ness'.

Originally from Latin the root concerned physical bristling or shuddering ('to tremble'), and over time the sense shifted to 'causing horror or dismay'; the noun now denotes that quality or state.