a small snake, often with marks in the shape of diamonds on its back. Adders are the only poisonous snakes in Britain. 宽蛇（英国仅有的一种毒蛇，背部有菱形斑） Word OriginOld English nædre ‘serpent, adder’, of Germanic origin; related to Dutch adder and German Natter. The initial n was lost in Middle English by wrong division of a naddre; compare with apron and umpire.