'anencephalus' originates from Greek, specifically from 'an-' + 'enképhalos' (ἐγκέφαλος), where 'an-' meant 'without' and 'enképhalos' meant 'brain'.

'anencephalus' was formed in New Latin/medical Latin from Greek elements (e.g. ἀν- + ἐγκέφαλος) and entered English usage in medical contexts in the 19th century as 'anencephalus'.

Initially, it meant 'without brain' (literally lacking the brain); over time it has been used specifically to denote a fetus or newborn with the congenital absence of major brain structures (the modern medical sense).