Excellent answer from Juan, i think. Daffy is a synonym for crazy. Crazy in Spanish means "loco". So "Daffy Duck" means "Pato Loco". But why "Lucas"? "Lucas" sometimes is used instead of "loco" in some informal spanish talking with the same meaning. So, if somebody tells you: "Estás Lucas" she is actually saying: "Estás loca". :D Regards!

Probablemente porque, de alguna manera, puede asociarse fonéticamente con "pato loco".