Other forms: wackos
Describing someone as a wacko is a derogatory way to say they're quirky or eccentric. It may be tempting to call the neighbor who walks his pet peacock on a leash a wacko, but it wouldn't be very nice.
You'll often hear someone use the word wacko as a way of dismissing people whose opinions or lifestyle they don't understand or agree with. If your cranky uncle complains about the wackos in town, then he's probably talking about a group of people he doesn't like very well. This informal noun first appeared around 1938, from wacky.