'unicephalous' originates from Latin and Greek: the prefix 'uni-' comes from Latin 'unus' meaning 'one', and the combining element 'cephal-'/'-cephalous' derives from Greek 'kephalē' meaning 'head' (via New Latin '-cephalus').

'unicephalous' was formed in New Latin as 'unicephalus' (combining Latin 'uni-' and Greek 'kephalē'), and this New Latin/Scientific formation was later adopted into modern English as 'unicephalous'.

Initially the elements separately meant 'one' and 'head'; over time the combined adjective came to be used in anatomical and zoological contexts to mean 'having a single head'.