What does adder mean?
The definition of adder
Add”er, n.
Etym: [See Add.]

One who, or that which, adds; esp., a machine for adding numbers.

Ad”der, n.
Etym: [OE. addere, naddere, eddre, AS. nædre, adder, snake; akin to OS. nadra, OHG. natra, natara, Ger. natter, Goth. nadrs, Icel. nathr, masc., nathra, fem.: cf. W. neidr, Gorn. naddyr, Ir. nathair, L. natrix, water snake. An adder is for a nadder.]

1 A serpent. [Obs.] “The eddre seide to the woman.” Wyclif. Gen. iii. 4. )
2 (Zoöl.)

(a) A small venomous serpent of the genus Vipera. The common European adder is the Vipera (or Pelias) berus. The puff adders of Africa are species of Clotho.
(b) In America, the term is commonly applied to several harmless snakes, as the milk adder, puffing adder, etc.
(c) Same as Sea Adder.
Note: In the sculptures the appellation is given to several venomous serpents, — sometimes to the horned viper (Cerastles).