'arrhythmous' originates from Greek, specifically from elements 'a-' + 'rhythmos', where 'a-' meant 'not' and 'rhythmos' meant 'rhythm' or 'measured flow'.

'arrhythmous' developed in English from Greek roots through Late Latin/medical formation such as 'arrhythmia' and the adjective-forming pattern (Greek/Latin root + -ous), yielding the adjective 'arrhythmous' as a variant of 'arrhythmic'.

Initially it meant 'not rhythmic' or 'without rhythm', and over time it has retained this core meaning as 'irregular in rhythm'.