“Old fool! Why, there is a large patch in the hollow of his left breast as bare as a snail out of its shell!”
"Old fool" is what the Lord of the Nazgul calls Gandalf, after destroying the gate of Minas Tirith. Gandalf calls Butterbur an old fool, and Rory Brandybuck refers to Bilbo as a silly old fool. Those are the only instances of "old fool" in The Hobbit and LOTR.
In the Rankin Bass animated "The Hobbit," Thorin flings the phrase at Gandalf, and Bilbo says it to Smaug, too.
Appropriately, Bilbo calls himself a fool only a few paragraphs later, after unwisely jeering at Smaug when taking leave of him.
