An anything but clothes party (ABC party) is a party where the dress code states that attendees can wear anything they wish as long as it is not clothing. This results in great creativity as people fashion all manner of objects into "clothing." It can also result in many attendees wearing very little.
Anything but clothes parties are most commonly seen on college campuses. These parties are sexy but also silly - the creative costumes people come up with make for great memories and photos.