according to the Word Detective, There's no particular logic to "cat got your tongue," except that cats have served as the object of human myth and metaphor for thousands of years. No sooner did the first caveperson open the door to a yowling cat than people began concocting stories about cats. The black ones bring bad luck. They have nine lives. They suck out your breath while you're sleeping. They make those mysterious long distance calls that show up on your phone bill.

The most surprising thing about "cat got your tongue" may be its relatively recent vintage. While it certainly sounds as if it must have been dreamt up back in the Middle Ages, the earliest written example listed in the Oxford English Dictionary is from 1911.

if you've got a better story, post it and we'll have at debunking it!