Merriam-Webster Collegiate Dictionary - adder

 I. noun  Etymology: Middle English, alteration (by false division of a naddre) of naddre, from Old English n?dre; akin to Old High German natara ~, Latin natrix water snake  Date: 14th century  1. the common venomous viper (Vipera berus) of Europe; broadly any of various snakes of the viper family — compare puff ~  2. any of several North American snakes (as the hognose snakes) that are harmless but are popularly believed to be venomous  II. noun  Date: 1580 one that adds; especially a device (as in a computer) that performs addition