'astheny' originates from New Latin, specifically the word 'asthenia', where the prefix 'a-' meant 'without' and Greek 'sthenos' meant 'strength'.

'astheny' developed from New Latin 'asthenia' (used in medical Latin in the 18th–19th centuries), ultimately tracing back to Greek 'asthenēs' (ἀσθενής), and entered English as an alternate/archaic form of 'asthenia' in the 19th century.

Initially it meant 'absence of strength' in the literal sense; over time it has remained largely the same but became specialized as a medical term referring to pathological weakness.