faŭko

One root, and it is faŭk-. There is no Esperanto morpheme faŭ- and no suffix -ko to peel off. Faŭko arrives whole from Latin and the only things built on it are built forwards: faŭki, to gape open, and alfaŭki, to shout at somebody with your mouth wide open. If a word starting faŭ is not one of those, it is a different root entirely.

Three senses, widening as they go. The first is an animal's: the open jaws and gullet of something large, a lion, a wolf, a crocodile. The second is mechanical: the mouth of a vessel, the opening of a bottle or a glass. The third is figurative and does most of the work in literature, a wide dark frightening opening, so a canyon, a mine shaft, a tunnel, a furnace, the muzzle of a cannon, and at the far end la faŭko de la morto, the jaws of death.

The fence with the neighbours. Buŝo is the mouth as an anatomical part, on a person or an animal, and it is the neutral word. Gorĝo is the throat, the passage behind it. Faŭko is neither: it is the mouth considered as a hole you could fall into. Using it of a person's mouth is an insult with a real edge to it, and fermu la faŭkon is the Esperanto for shut your gob.