adderyoudaoicibaDictgodict[adder 词源字典]
adder: [OE] In Old English, the term for a snake (any snake, not just an adder) was nǣddre; there are or were related forms in many other European languages, such as Latin natrix, Welsh neidr, and German natter (but there does not seem to be any connection with the natterjack toad). Around the 14th century, however, the word began to lose its initial consonant. The noun phrase including the indefinite article, a nadder, became misanalysed as an adder, and by the 17th century nadder had disappeared from the mainstream language (though it survived much longer in northern dialects).
[adder etymology, adder origin, 英语词源]
adder (n.)youdaoicibaDictgodict
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."