'asthenic' originates from Ancient Greek, specifically the word 'asthenēs', where 'a-' meant 'without' and 'sthenos' meant 'strength'.

'asthenic' changed from the Late Latin/Medieval Latin form 'asthenicus' (from Greek 'asthenēs') and eventually became the modern English word 'asthenic'.

Initially, it meant 'without strength' in a literal sense, and over time it has retained that core sense, becoming a medical adjective meaning 'lacking physical strength or vigor'.