Copper-bottomed is an adjective that means reliable.
Our word of the day combines two common English words ‘copper’ and ‘bottomed.’ In a literal sense it simply refers to something that is coated with copper — a very firm metal — on the bottom. But in a more metaphorical sense it refers to something that is solid like Copper and that comes with a guarantee.
I was told I had just purchased a copper-bottomed stock. As guaranteed, the stock shot up shortly after my buying it.