lowest part of the small intestine, 1680s, medical Latin, from ileum, singular created from classical Latin plural ilia "groin, flank," in classical Latin, "belly, the abdomen below the ribs," poetically, "entrails, guts." Sense restriction and form apparently from confusion with Greek eileos (see ileus). Earlier in English ylioun (late 14c.), from Medieval Latin ileon. Related: Ileitis.