kaduka

A thing about to fall down. The Latin behind kaduka is cadere, to fall, and the adjective caducus described what was ready to drop: leaves in autumn, a wall with nothing left holding it, a will that has lapsed. That is exactly the picture the Esperanto word carries. A kaduka building is not merely old, it is failing. The roof sags, the plaster has gone, and the sensible thing to do is stand somewhere else.

A word about who you use it on. In Esperanto as in English, this vocabulary is for buildings, bridges, boats, institutions and arguments. Applied to a person it is an insult, and a fairly cruel one: it says worn out and past use. If you want to say someone is old, the word is maljuna, which is neutral and carries no verdict at all; if they are frail, malforta or malsana describes the condition without passing sentence on the person. English speakers make this mistake with decrepit too, so the etiquette transfers directly - say it of the house, not of the owner.

Where it sits. Malnova is simply old. Ruino is what is left after the collapse. Putri is to rot, which is one way of getting there, and detrui is the active version, someone knocking the thing down. Kaduka names the state just before the end: still standing, but only technically.

Building on it. Kadukiĝi is to fall into decay, kadukeco the state of decay, and kadukulo - a person in that state - is available and best avoided for the reason above. The -iĝ- form is the useful one: La muro kadukiĝis is a whole English sentence in two Esperanto words.