'atonicity' originates from Modern English, specifically formed from the adjective 'atonic' plus the noun-forming suffix '-ity'; 'atonic' ultimately comes from Greek elements: the prefix 'a-' (Greek 'ἀ-') meaning 'not' and 'tonos' (Greek 'τόνος') meaning 'tone, tension'.

'atonicity' developed in English by adding '-ity' to 'atonic'. The adjective 'atonic' was formed from Late Latin/Medieval Latin and French adaptations of Greek 'tonikos' (from 'tonos'), and the negative prefix 'a-' from Greek was attached to indicate absence of tone; over time these parts fused into the modern English noun 'atonicity'.

Initially it meant 'absence of tone or tension' (from the Greek elements), and over time this core meaning has been retained, now used especially for lack of muscle tone in medicine or lack of tonal organization in music/linguistics.