<noun> Chiefly British Slang
1) To be one of a mentally diminished nature. See: [retard]
==> Term is a derivative of 'special' as some mentally challenged people are put in special areas of school and/or are known as "special people."
The term is used in a lighthearted sense to people who are not clinically disabled. Merely people who are known for doing absurd or stupid things.
> FLETCH: "That fucking Fisheye... He's such an idiot!"
> SCOTT: "Yeah he's such a fucking spesh."
> FISHEYE: "Huh huh, I'm making a brew, shag yer bird, shag yer bird! Huh huh!"