fekaĵo

Plain biology, plainly said. Feki at rank 1,605 is the neutral verb, the one a vet or a midwife uses, and -aĵ- turns a root into the concrete thing that comes of it. So fekaĵo is the substance: excrement, faeces, muck. It is what a doctor writes in a chart and what a gardener spreads on a field, and it belongs on this page in exactly that register.

The suffix is worth the detour. -Aĵ- makes a thing out of whatever it is stuck to: manĝi to eat gives manĝaĵo food, trinki to drink gives trinkaĵo a drink, nova new gives novaĵo a piece of news. Once you have it, dozens of words stop needing to be learned at all. The affixes article has the full set.

One warning about register. The dictionary gives a second, figurative sense: called of a person or a piece of work, fekaĵo is an insult of the obvious kind, and there it is as rude as the English equivalent. The literal sense is not. Keep the two apart the way you keep them apart in English, where a medical note and a shouted word can use the same root and land completely differently.

The neighbours. Feko is the same substance with the plain -o ending and is used interchangeably. Necesejo is the room, literally the necessary-place, which is one of the politest words any language has managed for it.