Basic Guidelines For English Spellings
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A robber, a plunderer, a looter.
Middle English. Probably partly (in early use) from Anglo-Norman ryflour, Middle French rifleur, riffleur robber, thief, and partly from rifle + -er. Compare post-classical Latin riflator, rifliator, riflerius (after 1341 in a British source).
Rifleman.
Late 18th century. From rifle + -er.