(Dr. ). A French physician in Shakespeare's Merry Wives of Windsor.
“The clipped English of Dr. Caius.” -Macaulay.
Caius College (Cambridge). Elevated by Dr. John Key (Caius), of Norwich, into a college, being previously only a hall called Gonville. Called Keys. (1557.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Caius from Infoplease: