Found this on anther site and wondered if there's any truth to it. Who else to ask but all you shantey singers/pirates?!

Some say that the expression "Cat got your Tongue" comes from the days of sailing ships, and refers to the cat of nine tails. If an officer told someone something in secrecy, he might warn that he would 'get the cat' if he was to tell the others. When others wanted to gain information from a silent crewmate, they might thus say 'Tell us, or has the cat got your tongue! )