1960, P. G. Wodehouse, Jeeves in the Offing, “Unquestionably his metabolism is unduly susceptible to stresses resulting from the interaction of external excitations,†he said, and Bobbie patted him on the shoulder in a maternal sort of way, a thing I wouldn't have cared to do myself though our relations were, as I have indicated, more cordial than they had been at one time, and told him he had said a mouthful.
"She sells sea shells" is a bit of a mouthful to say.
1960, P. G. Wodehouse, Jeeves in the Offing, “Yes, you may leave this little matter entirely to me, Mr Wooster.†“I wish you'd call me Bertie.†“Certainly, certainly.†“And might I call you Roderick?†“I shall be delighted.†“Or Roddy? Roderick's rather a mouthful.†“Whichever you prefer.â€
2010, Alexander Irvine, Iron Man 2: The Junior Novel, page 77"Tony, I'm the executive director of S.H.I.E.L.D., the Strategic Homeland Intelligence, Enforcement, and Logistics Division," explained Fury. Tony nodded. ... "Want a tip? Fire your namer of things, because that's a mouthful."
A tirade of abusive language (especially in the term "give someone a mouthful")