late 14c., from Old French ducat (late 14c.), from Italian ducato (12c.), from Medieval Latin ducatus "coin," originally "duchy," from dux (genitive ducis) "duke" (see duke (n.)).
So called for the name or effigy of Roger II of Sicily, Duke of Apulia, which first issued the coins (c.1140). Byzantine emperor Constantine X had the Greek form doux struck on his coins during his reign (1059-1067). Over the years it was a unit of currency of varying value in Holland, Russia, Austria, Sweden, Venice, etc. Remained popular in slang for "money" or "ticket" from its prominence in "The Merchant of Venice."