Festerius is some what of a minor internet celebrity that has been seen all over the internet since 1993. There are about 150 different refrences if you search for 'Festerius' on google.
Dude, did you see what that Festerius guy posted on our message board? Who is this weirdo?
What you say when did something stupid and you really have no explanation as why you did it in the first place. Comes from the psychopath John Hinckley, Jr. who tried to assassinate Ronald Reagan to impress Jodie Foster.
Cop: "Why did you throw your milkshake at the drive-thru lady?"