I'm a nerd. Yes, I'm in fact such a big nerd that I tend to look up word origins in my spare time because I'm fascinated by our language. The odder the origin, the better. I've got a collection of random information in my brain that makes me an awesome Trivial Pursuit partner, but is completely useless when it comes to real world application. Like say, job applications.
In any case, I thought I'd share some of this random crap with you in the form of the new acronym-ific series. I give you - Word Origins from Left Field. Er... ignore the fact that the "from" doesn't fit.
It's not something that's asked of me much, I'll admit.
It doesn't look like there is a dead-on answer for this one, as is the case with most idioms. However, there are some great, horrific possibilities.
The cat-o-nine tails was a nasty, nine-fingered whip with broken glass braided into it, or hooks attached to the ends. It was typically used on board ships to keep mouthy sailors in line. I suppose if my captain asked me if I had anything to say and he was holding on of those, I'd keep my mouth shut too.
Another reference I found was an ancient Middle Eastern practice of removing the tongues of liars and thieves and feeding them to cats.