A small lockbox meant for keeping valuable items safe…
A coffer is a small safe or lockbox that’s meant for keeping precious or valuable items safe. Your grandfather might keep his important papers in a coffer in his study.
You can keep your heirlooms, jewels, and savings bonds in a coffer, a safe chest for tucking such things safely away. Coffer takes on a figurative meaning when you talk about the coffers of a large institution: “There’s plenty of money in the government coffers — they just don’t want to spend it on education!” Yet another meaning of the word is “decorative sunken panel in a ceiling,” such as one you’d see in a mosque, museum, or cathedral ceiling.