A more thorough explanation:

Definition: A person who is mentally ill and poses a threat to themselves or others.

Example: John has been diagnosed with schizophrenia and has a history of violent outbursts. He is considered a dangerous lunatic and has been institutionalized for the safety of himself and others.

Explanation: The example illustrates the definition by showing how a person with a mental illness, in this case, schizophrenia, can become a danger to themselves or others. John's violent outbursts demonstrate the potential harm that a dangerous lunatic can cause, and the need for institutionalization to protect both the individual and society.