A disclaimer is a statement that rejects a legal claim. If you're speaking on Russian history and don't want to be quoted, offer a disclaimer that all your information comes from Wikipedia and you're not responsible for its accuracy.
A claim is the assertion that you have a right to something, so a disclaimer is the opposite: the rejection of a claim. If you don't want to keep the money you're inheriting, you can file a disclaimer and pass all the money on to a charity. Although disclaimer comes from talking about legal claims, we also use it in general to mean "denial" or "repudiation."