'arhythmically' originates from Greek, specifically from the negative prefix 'a-' and the noun 'rhythmos' (rhythm), where 'a-' meant 'not' and 'rhythmos' meant 'rhythm'.

'arhythmically' developed via Late Latin and Modern Latin forms such as 'arrhythmia' (from Greek arrhythmos) and then into English as the adjective 'arhythmical' / 'arrhythmic' and the adverb 'arhythmically' / 'arrhythmically'.

Initially it meant 'without rhythm' in a general sense, but over time it came to be used especially for 'irregular in timing or beat', notably in medical contexts referring to the heart.