1580s, "render (liquid) turbid or muddy by stirring up dregs or sediment," also figurative, "excite to some degree of anger, perturb," a word of uncertain origin. Perhaps it is 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" (see regulate). Or perhaps somehow imitative. An earlier borrowing of the French verb is Middle English roil "to roam or rove about" (early 14c.); also compare rile (v.), formerly the common colloquial form in the U.S. Related: Roiled; roiling.
updated on September 12, 2021