adder (n.) Look up adder at Dictionary.com
Old English næddre "a snake, serpent, viper," from Proto-Germanic *nædro "a snake" (cognates: Old Norse naðra, Middle Dutch nadre, Old High German natra, German Natter, Gothic nadrs), from PIE root *netr- (cognates: 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, nickname, humble pie, umpire. Nedder is still a northern English dialect form. Folklore connection with deafness is via Psalm lviii:1-5. The adder is said to stop up its ears to avoid hearing the snake charmer called in to drive it away. Adderbolt (late 15c.) was a former name for "dragonfly."