(2 syl.). Over the fur robe. (Latin, super-pellicium.) The clerical robe worn over the bachelor's ordinary dress, which was anciently made of sheepskin. The ancient Celts and Germans also wore a garment occasionally over their fur skins.
Durandus says: “The garments of the Jewish priesthood were girt tight about them, to signify the bondage of the law; but the surplice of the Christian priest is loose, to signify the freedom of the gospel.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Surplice from Infoplease: