'post-febrile' is formed from the Latin prefix 'post-' meaning 'after' and the Latin-derived adjective 'febrile' (from Latin 'febris') meaning 'fever'.

'post-' is a Latin prefix 'post' meaning 'after', and 'febrile' comes from Latin 'febrilis' (from 'febris' meaning 'fever'); the adjective 'febrile' entered English via Medieval Latin/Old French and combined with the prefix 'post-' in modern English to form 'post-febrile'.

Initially the components meant 'after' and 'fever'; combined as 'post-febrile' it has retained the sense 'occurring after a fever' in medical usage.