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1580s, of uncertain origin, probably from Middle French rouiller "to rust, make muddy," from Old French roil "mud, muck, rust" (12c.), from Vulgar Latin *robicula, from Latin robigo "rust" (see robust). An earlier borrowing of the French verb is Middle English roil "to roam or rove about" (early 14c.). Related: Roiled; roiling.