copper-bottomed

Having lower parts made of or covered by copper.

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'.

Thoroughly reliable; secure.

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