eu- combining form Etymology: Middle English, from Latin, from Greek, from ey, eu, from neuter of eys good; akin to Hittite asus good and perhaps to Sanskrit asti he is — more at is 1. a.: well : easily < euplastic > — opposed to dys- b.: good < eudaemon > — opposed to dys- 2. a.: most typical : true < Euascomycetes > < euchromosome > b.: truly < eucoelomate > c.: having a complete life cycle < eu-form > 3.: improved derivative of a (specified) substance < eucodeine >