When you lowball someone, you attempt to buy something from them by offering a lot less money than it's actually worth. If you want to buy a car that you can't quite afford, you might decide to lowball.
Whether you estimate something's value, make an offer to buy something, or try to hire a worker for a very small salary, you lowball. The word works as an adjective too: "He was selling fancy lemonade for two dollars a cup, but I made him a lowball offer, since I only had a quarter in my pocket." This informal slang word was coined in the US, from a railroad term.