An adder is a type of venomous snake, which is appropriate, because it has a poisoned etymology. Okay, maybe that's melodramatic, but it certainly took a surprising turn. Back in Middle English, the word was addere, and before that, it was naddere. Why the dropping of a consonant? Well, in the olden days, people said "a naddere" so much that it became "an addere," and the shift occurred (this is a process known as rebracketing). In Old English, the word was naeddre, and in Proto-Germanic, it was nadro. By then, the meaning had broadened to "a snake" in general, though, phonemically speaking, we have traveled quite a way from the original term. Further reconstructions take us to the Proto-Indo-European root netre, also "snake", which in turn may be from a word sounding like neh and meaning "to twist", because snakes twist. Apparently, there was a bit of natural selection going on: nadder was also a word, briefly, in the nineteenth century, before it faded into obscurity.
Adam Aleksic has been described as the internet's sixth most famous etymologist. He loves writing about himself in the third person, he's a freshman at Harvard University, and he has disturbing interests in linguistics, vexillology, geography, board games, limericks, and law.
The Etymology Nerd