Main Entry: [2]splat
Function: noun
Etymology: imitative
Date: 1897
: a splattering or slapping sound
If you mean why "splatbook" means a powergamer book, it's believed it comes from an old typesetter term for an asterisk, a 'splat.'
The books would be the Thieves' Book, the Barbarians' Book, the * Book... or a splat book.
I'm not aware of splat being an acronym, although I'm sure it is for something.
Hope that helps,
Greg