What does the expression to smell a rat?

The expression "to smell a rat" means to suspect that something is wrong or that someone is lying or behaving in a dishonest way. The phrase is based on the idea that rats are associated with dirt, disease, and dishonesty. When someone smells a rat, they are essentially detecting something suspicious or underhanded. The expression can be used in both formal and informal contexts. For example, someone might say, "I smell a rat in this deal," or "He smells a rat, but he can't quite figure out what it is."