'athetosic' originates from Greek, specifically the word 'ἄθετος' (transliterated 'athetos'), where the prefix 'a-' meant 'not' and 'thetos' (related to 'tithenai') meant 'placed'; the medical noun 'athetosis' plus the English adjectival suffix '-ic' produced 'athetosic'.

'athetosic' changed from the medical noun 'athetosis' (coined in the late 19th century) and eventually became the modern English adjective 'athetosic' by adding '-ic' to indicate 'pertaining to'.

Initially, it meant 'pertaining to the condition of athetosis'; over time it has retained that specialized medical meaning and remains chiefly descriptive in clinical contexts.