The word sniper obviously utilizes the -er suffix denoting a person; thus a sniper is one who has snipes. This is pretty close to the etymology; we all know that a snipe is also a word for a hard-to-catch bird, and in the olden days a sniper was a sharpshooter who was skilled enough to bust a cap in that particular kind of bird. In Middle English this word was snypa, and in Old English it was spelled snipa. This is from its Old Norse cognate snipa. This occasionally showed up in the form myrsnipa, which literally meant "moor snipe", and is of uncertain origin, though it is probably ubiquitous to Germanic languages and is definitely a word that traces from Proto-Indo-European (compare with the Latin genus name Scolopacidae). Usage of sniper only overtook snipe in the 1970s; current usage doesn't even come close to the peak of snipe in the 1800s.
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.
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