copper-bottomed

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.[1]

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copper-bottomed (comparative more copper-bottomed, superlative most copper-bottomed)

1. Having lower parts made of or covered by copper (especially of a ship or cookware).
2. (Britain, idiomatic) Thoroughly reliable; secure
   • 1807, Washington Irving, Salmagundi:[1]

     The copper-bottomed angel at Messrs. Paff’s in Broadway.

Related term copperfasten used with similar meaning, but different nuance – copper-bottomed means “reliable, trustworthy”, while copper-fastened means “secured, unambiguous”.[1]

• copperfasten

• coppered