1. (n) Facial hair that does not exist on the face, but instead on the neck. Almost never well groomed.
2. (n) Derogatory term for slovenly nerdy people who have no sense of hygene or grooming. Often related to hobbies such as card gaming, video gaming, anime, et. al.
If this party is full of f*cking neckbeards, I'm out. I've got better things to do than sit around with a bunch of unwashed nerds.