lowest part of the small intestine, 1680s, medical Latin, from ileum, in medieval medicine "the part of the small intestines in the region of the flank," singular created from Latin ilia (pl.) "groin, flank," in classical Latin, "belly, the abdomen below the ribs," poetically, "entrails, guts." The word apparently was confused in Latin with Greek eileos "colic" (see ileus), or perhaps is a borrowing of it. The sense is "winding, turning," either via the Greek meaning or from the convolutions of the intestines. Earlier in English ylioun (late 14c.), from Medieval Latin ileon. Related: Ileitis; ileal.