'posterior-medial' is a modern English compound formed from the adjective 'posterior' + the adjective 'medial'. 'posterior' originates from Latin, specifically the word 'posterus' (from 'post' meaning 'after' or 'behind'), and 'medial' originates from Latin, specifically the word 'medialis' (from 'medius' meaning 'middle').

'posterior' comes from Latin 'posterus' and entered English via Late Latin and Old French/Medieval Latin forms; 'medial' comes from Latin 'medialis' (from 'medius') and entered English via Late Latin/Old French. The compound combining these elements into anatomical directional phrases (e.g., 'posteromedial' or hyphenated 'posterior-medial') developed in modern medical English usage.

Individually, 'posterior' originally meant 'coming after' or 'at the back' and 'medial' meant 'pertaining to the middle'; together they have maintained the combined directional meaning 'toward the back and the midline' in anatomical contexts.