A venomous snake found in Europe and Asia, or a person who adds things together.
From Old English 'nædre' meaning snake, which came from Proto-Germanic roots. The 'n' was lost over time through a process called reanalysis, where 'an adder' was misheard as 'a nadder' becoming 'an adder.' The math meaning came much later from the verb 'add.'
The loss of the 'n' in 'nadder' is the same linguistic accident that created 'an orange'—originally 'une narange' in French got reanalyzed by English speakers who heard the wrong word boundary!
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