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.
Adjective. Older than old school. Describing some experience, action, or word to which someone under sixty years-old would not likely have been exposed or no longer has relevence.
Fossil move dude - who let you out of your tar pit?. How fossil! Mondo fossil. Way fossil.