'atrociousness' originates from English 'atrocious' + the noun-forming suffix '-ness', where 'atrocious' comes from French 'atroce' and ultimately from Latin 'atrox' meaning 'cruel' or 'fierce'.

'atrox' (Latin) → 'atroce' (Old/French) → 'atrocious' (Middle/Modern English adjective); the noun 'atrociousness' was formed in English by adding the suffix '-ness' to the adjective 'atrocious'.

Initially the Latin 'atrox' meant 'cruel' or 'fierce'; over time the sense shifted through Old French to English where 'atrocious' came to mean 'extremely bad, horrifying, or cruel', and 'atrociousness' denotes that quality or degree.