Old English næddre "a snake, serpent, viper," from West Germanic *nædro "a snake" (cf. Old Norse naðra, Middle Dutch nadre, Old High German natra, German Natter, Gothic nadrs), from PIE root *netr- (cf. Latin natrix "water snake," probably by folk-association with nare "to swim;" Old Irish nathir, Welsh neidr "adder").
The modern form represents a faulty separation 14c.-16c. into an adder, for which see also apron, auger,