ducat (n.)
late 14c., from Old French ducat (late 14c.), from Italian ducato (12c.), from Medieval Latin ducatus "coin," originally "duchy," from dux (genitive ducis) "duke," from PIE root *deuk- "to lead."

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."