‘Bottom fishing’ is a term used in the stock market. When you go bottom fishing, you look for shares whose prices have dropped very low because of a crash in the market.
The term has acquired a broader meaning. When you go bottom fishing, you try to buy something at the cheapest possible price. And in the process are willing to settle for products of poor quality.
Examples: ”I have a cousin who thinks that bottom fishing will always get you something of value”. ”I don’t think bottom fishing is always wise. There is a reason why the seller is willing to sell something at such a low price.”