Ad'der, the only poisonous snake of Great Britain. The name, however, is generally applied to the viper and to almost any poisonous snake. The name is derived from the serpent's power of puffing out the upper part of its neck when irritated or alarmed. The puff adder or asp is a snake of South Africa whose bite is always fatal. It is very thick and attains a length of four or five feet. The natives poison their arrows with its venom.