pisi

The plain word, and it is plainer than you expect. English speakers arrive at pisi braced, because the obvious English cognate is coarse. Esperanto's is not. The Reta Vortaro gives pisi no style marking at all: it is the ordinary word, the one a parent says to a toddler and a vet says about a dog. It is informal rather than clinical, and it is not obscene. This is the same situation as feki at rank 1,605, and for the same reason - Esperanto took the root and the verb ending and did not import the English word's social history along with it.

The two registers, and when each is right. Pisi is the everyday verb. Urini is the formal one, and it is what a doctor writes: urino is urine, urina the adjective, urinejo the urinal. If you are describing a symptom to a nurse, use urini. In any other sentence pisi is normal and urini sounds like a form being filled in. The noun from this page is piso, the act, and pisaĵo is the substance; pispoto is a chamber pot, mostly for small children, and it is a real dictionary word rather than a joke.

Knowing it precisely saves you groping for a euphemism. That is the whole argument for a page like this one. Half the reason learners avoid these verbs is that they are not sure how rude they are, and the uncertainty costs more than the word does - you end up circling a puppy, a nappy or a long car journey with vague phrasing when one short verb would do. The room is necesejo. The sign on the door is a picture. And if you want the vulgar register in Esperanto, that lives elsewhere, in a small set of interjections that have nothing to do with this verb.