mid-15c., "light-colored wine," from Old French (vin) claret "clear (wine), light-colored red wine" (also "sweetened wine," a sense in English from late 14c.), from Latin clarus "clear" (see clear (adj.)). Narrowed English meaning "red wine of Bordeaux" (excluding burgundy) first attested 1700. Used in pugilistic slang for "blood" from c.1600.
[1604+ Prizefight; fr the red color of claret wine]