'posteroinferior' originates from modern medical (Neo-Latin) usage combining Latin elements: 'poster(o)-' from Latin 'posterior' meaning 'behind/after' and 'inferior' meaning 'lower'.

'posteroinferior' was formed in modern anatomical/medical English as a compound of 'posterior' + 'inferior' to describe a position that is both behind and below within a body or structure.

Initially, the constituent parts separately meant 'behind' and 'lower'; over time the compound came to be used as a single adjectival term meaning 'toward the back and lower part' without implying two separate directions.