Arthritic is the word we use when we talk about arthritis, a disease that makes the joints — the places where bones meet, like knees, fingers, hips — swell, stiffen, and hurt. The pain can be worse in the morning or in cold weather, and the joint may look puffy. Doctors call the illness arthritis; arthritic describes a person who has it, or a joint that is suffering from it: arthritic knees, an arthritic patient. As a noun, “an arthritic” means a person living with that illness; some people now prefer “a person with arthritis”, because a person is more than their pain. Informal English also stretches it: a creaking old door might be called “arthritic” as a small joke, as if the door had sore joints. That joke can sound unkind if you aim it at a person. The medical use is serious and a little formal. Older people hear it more, because the illness is more common with age — though actually young people can have it too.