Adder comes from the Old English word nædre, ‘serpent’, from a Germanic root: the n was lost in medieval times through a naddre being read as an addre. Other words formed by a similar process of wrong division include apron, originally a napron, and auger (a nauger). The process sometimes works the other way, resulting in n being added to the beginning of a word: a newt was originally an ewt, a nickname an eke-name, and an umpire a noumpere.