Use the adjective rotten to describe something that is decaying or decayed. If you are like most people, you occasionally have to throw out rotten food — sometimes, it's so nasty you can't even tell what the food was in its original form!
The adjective rotten also describes something that is very poor in quality. If your car has a flat tire, your dog threw up on the carpet, and your boss yelled at you, you can say you are having "a rotten day." Another use for rotten is to describe something that is damaged by decay. Weather damage can cause boards to rot, so if you are working on an old house, you should especially watch out for rotten floor boards. You wouldn't want to fall through to the basement!