If a plan, a promise, or a person is copper-bottomed, you can depend on it — it is solid, safe, not a gamble. This is especially British English; American speakers are more likely to say rock-solid or watertight. Though actually it began as a real ship fact, not a metaphor. In the 1700s, wooden ships had copper sheets fixed to the bottom so sea worms and plants would not eat the wood. A copper-bottomed ship lasted. Britain had a huge navy, so there was always a market for copper in shipbuilding, and copper felt like a safe thing to put money in. From at least 1807 people also used the phrase in a picture way, about trust, not only about boats. In a kitchen, a copper-bottomed pan can still mean a pan with a copper base — but in news and business English it almost always means “reliable.”