'diabolical' originates from Late Latin, specifically the word 'diabolicus', which itself comes from Greek 'diabolos' where 'dia-' + 'ballein' (or the compound) conveyed the sense of 'slanderer' or 'one who throws across (an accusation)'.

'diabolical' changed from Greek 'diabolos' to Late Latin 'diabolicus', passed into Medieval Latin and Old French forms (e.g. 'diabolique'), and eventually became the modern English word 'diabolical' (from which 'diabolically' is formed).

Initially, it meant 'of or relating to the devil' (i.e. demonic or accusing), but over time it broadened to mean 'extremely wicked' and by extension 'extremely' or 'remarkably' in figurative uses (as in 'diabolically clever').