For purposes of modern
slang, the word was originally developed as a combination of the
words COCK and CUNT for the radio show "Taste of the Goods." CUNK sounded too much like CUNT for radio. CANK made a better sound for a non gender specific insult involving the genetalia. The final meaning of "cank" is dumb, and doesn't speak (as when arrested), and this stems from old english theives
slang. It is also exactly what happens when someone has a cank in their mouth.