'atonic' originates from Ancient Greek, specifically from the prefix 'a-' (Greek ἀ-, meaning 'not') combined with 'tonos' (Greek τόνος, meaning 'tone' or 'tension').

'atonic' changed from Late Latin/medical Latin 'atonicus' (formed from Greek elements) and entered English via medical Latin/early modern usage to become the modern English 'atonic'.

Initially it meant 'without tone' in the literal sense (absence of tension or pitch); over time it has been used specifically for 'lacking muscular tone' in medicine and 'lacking stress or pitch' in phonetics.