-An old fashioned kettle which was heated by fire
-Fires are up out by firemen
-Firemen give you a thumbs up if you give them a thumbs up
-On a computer if you type a thumbs up you get (Y)
-Which looks a bit like a cunt.
So the term fucking kettle is basically calling someone a fucking cunt.
person 1 - Hey did you see that indie kid?
Person 2 - What a fucking kettle