A "Lonely Larry" is a person who is not included or is rejected to a group. He/She is usually seen by themselves at parties, games, etc.
This term is derived from a person named Coleman Bauer. He was referred to as a "Lonely Larry" while being seen to be by himself while others were enjoying a great time. The originator is most likely Carson Ciamaichelo, his arch-nemesis.
Isaiah: Hey, Carson look at that kid over there...
Carson: Yeah, what about him?
Isaiah: What a Lonely Larry!