'arrhythmic' originates from Ancient Greek, specifically the word 'arrhythmikos', where the prefix 'a-' meant 'not' or 'without' and 'rhythmos' meant 'rhythm'.

'arrhythmikos' entered scientific/medical Latin and later Middle French/English medical vocabulary as 'arrhythmic' (and related forms), and 'arrhythmically' developed as the adverbial form in modern English.

Initially it meant 'without rhythm' in Greek; over time it retained the core sense and became a medical/descriptive term meaning 'lacking a normal or regular rhythm' in modern usage.