'posterior-inferior' is a compound of two Latin-derived English adjectives: 'posterior' (from Latin 'posterus') and 'inferior' (from Latin 'inferus').

'posterior' comes from Latin 'posterus' meaning 'coming after' and entered English via Late Latin/Medieval Latin and Old French as 'posterior'; 'inferior' comes from Latin 'inferus'/'inferior' meaning 'below' and passed into English through Medieval Latin/Old French as 'inferior'. The two terms were combined in scientific and anatomical usage to form the compound 'posterior-inferior'.

Originally 'posterior' carried the sense 'coming after' and 'inferior' meant 'lower'; over time, particularly in anatomy, both took on spatial directional senses ('toward the back' and 'below'), and together they now denote 'back and below' in descriptive anatomical language.