To buy a lemon means to buy an automobile that is always giving problems, so implying that the purchase was a waste of money. It refers to the fact that if you do not buy a good lemon, it will give a very sour taste. This idiom is used only in reference to a vehicle and no other goods or services.
1. After five miles of cruising on the highway, the car's engine failed and the motorist realized that he had bought a lemon.
2. Jacob happily bought the sedan from his cousin as he had to pay only half the cost, but got angry with the constant breakdowns and realized that he had bought a lemon.
3. Shivani realized that the car salesman had conned her into believing the four-door saloon was an economical vehicle; she was sure that she had bought a lemon when she saw the steady rise of fuel and repair bills.