The word is attested in Classical Latin, but is Greek in origin (ορεξις). Galen of Pergamon originally wrote his medical treatises in Greek, although they circulated in Latin translation in medieval times.
s.
1appetite, craving:
(
c.1300; MS: s.xivin
)
Fastidium est apelé de Galien anorexia, si est dit de a, ke est a dire 'sanz', e orexis, ke est a dire 'appetit', sicum 'sanz appetit'
ii 168.133
orexis (c.1300)