A lunatic is someone who behaves in a wild, irrational, or crazy way. The word comes from the Latin word for moon (luna), because people once believed that the moon's phases could make people lose their minds or act strangely.

Today, lunatic is mostly used informally to describe someone doing something ridiculous or reckless. You might call your friend a lunatic for attempting a dangerous skateboard trick, or describe a lunatic plan to stay up all night finishing a project that's due the next morning. When used this way, it's usually exaggerated and not meant literally: you're not actually saying the person has lost their mind, just that their choice seems wild or foolish.

In older times, lunatic was a serious medical and legal term for people with mental illness, which is why you'll encounter it in historical documents or classic literature. But that usage is outdated and considered offensive today. Mental health conditions are now understood scientifically and described with respectful, accurate language.

The phrase lunatic fringe refers to the extreme, unreasonable edge of any group: the people whose views are so radical or bizarre that even others in their group think they've gone too far.