'fiendishness' originates from English, specifically formed from the adjective 'fiendish' (from 'fiend') plus the noun-forming suffix '-ness'; 'fiend' comes from Old English 'feond', meaning 'enemy' or 'adversary'.

'fiendishness' developed from Middle English formations: Old English 'feond' became Middle English 'fiend' (meaning 'enemy' and later 'demon'), then the adjective 'fiendish' was formed with '-ish', and finally the noun 'fiendishness' was created with '-ness'.

Initially 'feond' meant 'enemy' or 'adversary'; over time the sense shifted toward 'demon' or 'evil spirit', and the modern derivative 'fiendishness' now denotes extreme cruelty, wickedness, or devilish cleverness.