Lousy things are terrible. The lousy things in life are the ones you complain about: your lousy job, the lousy weather, and the lousy pizza you had for lunch.
The slang word lousy is the perfect way to describe something particularly awful or rotten. When you're sick, you feel lousy, and when someone is mean or rude, they treat you in a lousy way. The original, literal meaning of lousy is "infested with lice," those creepy crawly parasites that hang out on people's scalps. It was once common to say a place was "lousy with" something (like tourists or poodles) to mean "swarming with" them.