From the copper sheathing applied to the bottom of a wooden ship to prevent damage from marine organisms. In literal sense 18th century, in figurative sense attested since at least 1807.
copper-bottomed (comparative more copper-bottomed, superlative most copper-bottomed)
- Having lower parts made of or covered by copper (especially of a ship or cookware).
- (Britain, idiomatic) Thoroughly reliable.
Related term copperfasten used with similar meaning, but different nuance – copper-bottomed means “reliable, trustworthy”, while copper-fastened means “secured, unambiguous”.