Ileum, from the Greek word for twisted, elios. Its a matter of conjecture why only the last 2/3s of the small intestine should be called "twisted" (in the sense of meadering) even though the initial 1/3, the jejunum, is worthy of the distinction as well.

It has been suggested that the word may stem from the use of elios by ancient Greeks for pathologies such as torsion, obstructions, or paralysis that are somewhat more common in the more distal portions of the small intestine .

Ileus as a term for intestinal paralysis was first used by Hippocrates in the 5th century B.C.

The ilium of the pelvic girdle didn't make its appearance in anatomy until Vesalius named it in the 16th century. The ancient Romans used the word for the soft areas of the abdomen, such as the flank or belly, and not the bone; in fact it may have come from ilia, the Latin word for soft (ilia was used by the Greeks for the female external genitalia). Others have suggested it comes from os ilei, "the bone the ileum lies against".