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.