kako

The register is the whole point of this entry. Esperanto has a small ladder of words for the same substance and they are not interchangeable. Kako is the nursery word - what a parent says to a two-year-old, what a paediatrician says to a parent. Fekaĵo, at rank 2,824, is the clinical and literary noun, faeces or excrement, and it is what you would write in a medical note or a field report. Ekskremento is the technical term proper. And feki, at rank 1,605, is the plain verb, to defecate. A learner who knows only one of these will sooner or later use it in the wrong room.

Why a childish word earns a place in a frequency list. Because it is the one you actually need first. The vocabulary of small children, illness and the necesejo is the vocabulary of daily life with a bebo, and it is the part of a language that textbooks skip and travellers need at three in the morning. A course that covers kometo and not kako is a course written for the wrong reasons.

Grammar and family, briefly. It is a regular noun: kako, kakon, kakoj. The verb kaki is to do a poo, again on the nursery register. Kakujo would be a container for it, and the polite compound kakejo is one of several words for a lavatory, though necesejo is the standard one.

Two near-collisions to keep straight. Kako is not kakao, which is cocoa, and one careless vowel turns a hot drink into something else entirely. And the verb kaki is not the colour: khaki is kakia, from a separate root that Esperanto took from Urdu by way of English, and it keeps its i all the way through. Say each of the three deliberately and the trouble goes away.