Rectum
mid 16th century: from Latin rectum (intestinum) ‘straight (intestine)’.
wiktionary
Borrowed from New Latin rectum, clipping of Latin rectum intestinum(literally “the straight intestine”), rectum, neuter of rectus(“straight”). See right.
etymonline
rectum (n.)
in anatomy, "the terminal section of the intestine, ending in the anus," early 15c., from Latin intestinum rectum "straight intestine" (in contrast to the convolution of the rest of the bowels), from neuter past participle of regere "to keep straight" (from PIE root *reg- "move in a straight line"). A loan-translation of Greek 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].
