'anencephalous' originates from New Latin, ultimately from Greek 'anencephalos', where 'an-' meant 'without' and 'enkephalos' (or 'kephalos' in components) meant 'brain' or 'head'.

'anencephalos' from Greek passed into Late Latin/New Latin as 'anencephalus' and was adopted into English in scientific/medical usage as 'anencephalous' in the 19th century.

Initially it meant 'without a brain' and over time has retained this specific medical meaning of 'lacking a major portion of the brain (congenitally)'.