E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Round as Giottos O. An Italian proverb applied to a dull, stupid fellow. The Pope, wishing to obtain some art decorations, sent a messenger to obtain specimens of the chief artists of Italy. The messenger came to Giotto and delivered his message, whereupon the artist simply drew a circle with red paint. The messenger, in amazement, asked Giotto if that were all. Giotto replied, Send it, and we shall see if his Holiness understands the hint. A specimen of genius about equal to a brick as a specimen of an edifice.