'asthenia' originates from Modern Latin 'asthenia', ultimately from Greek 'asthenēs', where the prefix 'a-' meant 'without' and 'sthenos' meant 'strength'.

'asthenēs' (Greek) passed into medical/Neo-Latin as 'asthenia' and was adopted into English in the 19th century as 'asthenia'; the plural form 'asthenias' follows regular English pluralization.

Initially it referred in Greek/medical contexts to 'lack of strength' or 'weakness'; this central meaning has been retained in modern medical usage.