A person, typically a no life, college-aged, high school graduated degenerate, who regularly attends high school parties hosted by those who are still in high school.
High School student 1 - Why is John at this party? Didn't he graduate from high school in 2007?
High School student 2 - He did graduate in '07. Man what a fossil.
FOSSIL is a standard for allowing serial communication for telecommunications programs under DOS. FOSSIL stands for Fido Opus Seadog Standard Interface Layer and was made by a group of Fidonet sysops to allow software work on different machines.