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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, 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."
(Ps. 140:3; Rom. 3:13, "asp") is the rendering of, (1.) Akshub ("coiling" or "lying in wait"), properly an asp or viper, found only in this passage. (2.) Pethen ("twisting"), a viper or venomous serpent identified with the cobra (Naja haje) (Ps. 58:4; 91:13); elsewhere "asp." (3.) Tziphoni ("hissing") (Prov. 23:32); elsewhere rendered "cockatrice," Isa. 11:8; 14:29; 59:5; Jer. 8:17, as it is here in the margin of the Authorized Version. The Revised Version has "basilisk." This may have been the yellow viper, the Daboia xanthina, the largest and most dangerous of the vipers of Palestine. (4.) Shephiphon ("creeping"), occurring only in Gen. 49:17, the small speckled venomous snake, the "horned snake," or cerastes. Dan is compared to this serpent, which springs from its hiding-place on the passer-by.