Dustin Hoffman on Graham Norton

He mentioned "television's most taboo word" apparently

Dustin Hoffman told his favourite joke on Graham Norton’s show recently, which included “television’s most taboo word”.

Irrespective of the joke - which I’d like to hear - I mostly want to know which word can least be said on TV.

Is it cunt? Motherfucker? Baby-squicking mother-felcher? (If not the latter, why not?)

According to the BBC, someone has been rapped on the knuckles for the joke. But what the hell was it - does anyone know?