This expression is used when someone seems to have been rendered speechless by a question or accusation. Ask Yahoo says there is no certainty about the origin, and the three guesses are all pretty disgusting. One is that liars had their tongues taken out and fed to the king’s cats in the Middle East. Another is that fear of being whipped with a cat-o-nine-tails renders a victim speechless. The final is that witches in the Middle East were feared and put to death. The cats of witches would control your tongue so you couldn’t report her.