'asthenical' originates from Greek, specifically the word 'asthēnēs', where the prefix 'a-' meant 'without' and 'sthénos' (root) meant 'strength'.

'asthenical' changed from the Greek word 'asthēnēs' to Late Latin/Medieval Latin forms such as 'asthenicus' and then entered English via New Latin/French influence as 'asthenic' and later 'asthenical'.

Initially it meant 'without strength' (literally lacking strength); over time it has kept that core sense and is used to mean 'weak, debilitated, or lacking vigor' in both medical and literary contexts.