'atonic' originates from Greek, specifically the word 'atonos', where 'a-' meant 'without' and 'tonos' meant 'tone' or 'stretch'.

'atonic' changed from the Greek word 'atonos' into the Late Latin/medical Latin term 'atonicus' and eventually became the modern English word 'atonic' through scientific/medical usage.

Initially it meant 'without tone' in a literal sense; over time it developed into the technical senses 'lacking physiological muscle tone' and 'unstressed' in phonetics.