copper-bottomed

Meanings

1. Having lower parts made of or covered by copper.
2. Thoroughly reliable; secure.

Etymology

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. 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.