When you want one word to describe being completely fed up and having entirely had enough of whatever is annoying you. Used to imply more emphasis than than the individual words alone can do. Used in a similar way to a-whole-nother or ri-fucking-diculous. This could also be spelt "e-total-nuff" in a less formal and possibly more satisfying manner!
Everyone was being too noisey, I had just had e-total-nough!