postaĵo

A clean build with a delicate job. Post is after or behind. The suffix -aĵ- turns an abstract idea into a concrete thing. So postaĵo is literally the behind-thing: the back part of whatever you are talking about.

That is the primary sense and it is entirely neutral. The postaĵo of a house is its rear, the postaĵo of a ship is its stern, the postaĵo of a queue is the end of it. English does the same thing with the back and nobody snickers.

And then there is the other use. Because English did it too, and French (le postérieur) and German (das Hinterteil) and half the languages in Europe: pointing vaguely at "the back part" is how polite speech has always handled the buttocks. Postaĵo is the euphemism, and it is the one you use in mixed company, in a doctor's surgery, and in writing.

The register fence, which is what this page is really for.

• postaĵo - polite, slightly formal, safe everywhere. Roughly English posterior or behind.
• pugo at rank 1,555 - the blunt anatomical word. Not obscene, but plain to the point of rudeness in the wrong room. Roughly English bum or arse, depending on how charitable your reader is feeling.
• sidvango - literally sit-cheek, a single buttock, and the term anatomy books use.
• postaĵeto on a child is affectionate, the -et- doing its usual softening.

The fact that pugo outranks postaĵo by a thousand places in our frequency list says something about the corpus rather than about politeness: the blunt word turns up in comic and colloquial writing, and the polite one in careful prose, and there was more of the former in the public-domain texts the list is built from.

The wider post- family. Posta is the adjective (later, subsequent), poste means afterwards, postulo is unrelated despite appearances, and postmorta is post-mortem. The one to watch is poŝto, the mail, which is a different root altogether and wears a hat that post never does.