1911 Encyclopædia Britannica/Dago

DAGO, a name given somewhat contemptuously to Spanish, Portuguese and Italian sailors, as “Dutchman” is similarly applied to Germans and Scandinavians as well as to natives of Holland. In America the word is generally confined to the poorer class of Italian immigrants. In the South Wales mining districts the casual labourers, who are only engaged when work is plentiful, are so called. The word is apparently a corruption of the common Spanish and Portuguese Christian name “Diego.”