1540s, from L. intestinum rectum "straight intestine," in contrast to the convolution of the rest of the bowels, from neut. pp. of regere "to straighten" (see right). A loan-translation of Gk. apeuthysmeon enteron, "the name given to the lowest part of the large intestine
by Galen, who so called it because he dissected only animals whose rectum (in contradistinction to that of man) is really straight" [Klein].