Dear mav: marvelous. My dictionary has a small difference: it says from "ileum", bowel, not "ilium" = pelvic bone. Obviously there must be an etymological link between "ileum"=bowel, and "ilium"=pelvic bone that I had never thought of.

5Fr < Sp (piedra de) ijada, (stone of) the side, loin < VL *iliata < L ilia, pl. of ileum (see ILEUM): from the notion that it cured pains in the side6