'postero-internal' is a modern English compound formed from the combining form 'postero-' (from Latin 'posterus' meaning 'coming after, posterior') and 'internal' (from Latin 'internus' meaning 'inward').

'postero-' derives from Latin 'posterus' -> Medieval Latin forms and later combining use in anatomical English; 'internal' comes from Latin 'internus' via Old French/Medieval Latin into Middle English, and the two elements were combined in modern anatomical terminology to form 'postero-internal'.

Originally, the roots meant 'coming after' (posterus) and 'inward' (internus); together in modern usage they specifically denote a location that is both toward the back and the inner/medial side of a structure.