A person you think is crazy, strange, or dangerous in their thinking. It is a rude, informal word, used to push someone away: 'Don't listen to him, he's a wacko.' It is often used about people with extreme beliefs — 'some wacko sent the office a threatening letter', 'conspiracy wackos'. The word can also work as an adjective: 'a wacko idea', 'wacko theories'. Note the difference from 'wacky', which comes from the same root but feels completely different. 'Wacky' is affectionate — a wacky friend is fun. 'Wacko' is not affectionate — a wacko friend is someone you are worried about, or someone you are insulting. Two more things. First, this is very much American English; British speakers know it but use 'nutter' or 'nutjob' more. Second, and more important: because it points at mental illness, careful speakers now avoid it. In a newspaper, a school, or any polite setting, calling a real person a wacko sounds cruel and old-fashioned. Understand it when you hear it; think twice before saying it.