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] The British idiomatic use comes from the fact that with Britain being a major naval power, there was always a ready market for copper in ship construction, making it a reliable commodity to invest in.

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

1. (literal, especially of a ship or cookware) Having lower parts made of or covered by copper.
   • 2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 88:

     According to Robert Hulse, 'It's half worm, half mollusc, and there are more dead men at the bottom of the sea as a result of those things [shipworms] than all the naval battles put together. They're the reason you had copper-bottomed ships'.

2. (British, 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
• gilt-edged