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