'posterointernal' originates from New Latin (medical formation), combining the element 'postero-' (from Latin 'posterus') and the Latin adjective 'internus'; in this combination 'post-'/ 'poster-' meant 'after, behind' and 'internus' meant 'inward, inside'.

'posterointernal' developed from compounds in Medieval/Medical Latin such as 'postero-internal' (a hyphenated combining form) and was adopted into modern English usage as the single-word adjective 'posterointernal'.

Initially the parts signified 'after/behind' + 'inward/inside'; over time the compound came to be used specifically as an anatomical directional term meaning 'located toward the back and inner side'.