/pɒˈstɪəriər/
[adjective] Coming after something in time or position; situated at or towards the back. In informal use, it is also used as a noun to refer to a person's bottom.
The word 'posterior' is the comparative form of a Latin word meaning 'after' — so it literally means 'more behind'.
The story of 'posterior' begins in ancient Latin with the simple preposition and prefix 'post', meaning 'after' or 'behind'. This small but powerful word gave Latin speakers a way to express position in both time and space — what comes after, what lies behind. From 'post', Latin formed the adjective 'posterus', meaning 'coming after' or 'following'. It was a natural extension of the spatial sense of 'post' into a more general idea of sequence and order.
From 'posterus', Latin then created a comparative form — 'posterior' — which meant 'more behind' or 'coming later'. In Latin grammar, comparative adjectives express a greater degree of a quality, so while 'posterus' meant simply 'coming after', 'posterior' meant 'further back' or 'more subsequent'. This comparative form is the direct ancestor of the English word we use today, arriving essentially unchanged in both spelling and meaning from the classical Latin original.
The word entered English in the early 16th century, carried by scholars and writers who were deeply immersed in Latin texts. In its earliest English uses, 'posterior' appeared in learned and medical writing, where it described parts of the body located towards the back, or events and ideas that came later in time. The medical world in particular found it useful — describing the rear-facing surfaces of organs or the back portions of anatomical structures, a usage that continues in medical English to this day.
Interestingly, Latin 'post' also gave English a remarkable family of related words through different routes. The Latin 'posterus' produced 'posteri', meaning 'descendants' or 'those who come after', and from this came the English word 'posterity' — meaning future generations. Meanwhile, 'post' itself, in the sense of 'after', passed through Medieval Latin and Old French into English in words like 'postpone' (Latin 'postponere': 'post' + 'ponere', to place — meaning to place after), and 'postscript' (Latin 'postscriptum': written after).
The informal use of 'posterior' as a polite or humorous noun for the buttocks developed later, as English speakers found the clinical, Latin-flavoured word a useful euphemism — a way of referring to the backside without using a more blunt term. This kind of borrowing from formal vocabulary for gentle comedy is a long tradition in English, and 'posterior' fits the pattern perfectly: it sounds impressively learned while describing something very ordinary. Today the word comfortably spans both worlds — rigorous anatomical description in a medical textbook and gentle wit in everyday conversation.