Derived from the (Y) sign used on instant messaging, as this looks slightly like a vagina. So, when you thumbs up someone, you're calling them a cunt. Taking this further, when a fireman thumbs up you, he's calling you a cunt. Kettles were originally heated by fire, which is put out by firemen, so Kettle became slang for cunt.
she had a right bucket of a kettle!