Well, first there where many books from multiple companies called something book (clanbook, worldbook, sourcebook, classbook, etc.). Then somewhere, not sure, thhey starting using *book as a way to through all these types into one general catagory. The * is called a Splat by a certain group of people so they are called splat books.
I'm sure someone knows the story much better, but that should get you the basic idea.
The sound they make when you slam them down on the table...softcovers SPLAT nicely.
Or....
The fact that the information within seems to have originated from a burst and spread, like a splat. For instance, sword and fist took the idea of Fighters and Monks, and blew it up -- SPLAT!
At least, that's always been my association.
Of course, when you're doing a splatbook, it must be crunchy....cruchy splats that are not shafted.
The '*' is called a 'splat' by publishers and those in the publishing industry. It was just picked by gamers as a derogatory term for anything they don't like.
I thought splatbook meant that a gaming company publishes a bunch of non-core book supplements and hope they make a "splat" with the gamers. The stuff in the book isn't "A" material but some segment of the gaming population my like it.
Sort of a long the lines of "throw stuff at it until something sticks."