'arhythmical' originates from Greek, specifically from the prefix 'a-' (meaning 'not' or 'without') plus Greek 'rhythmos' meaning 'rhythm'.

'arhythmical' passed into Late Latin/Medieval Latin as 'arrhythmicus' (from Greek 'arrhythmikos') and then into English as 'arrhythmical'/'arhythmical' (modern forms).

Initially it meant 'without rhythm' in a literal sense; over time it has been used particularly in medical contexts to describe an 'irregular heartbeat' but retains the broader sense 'not rhythmic'.