perineum n.

also perinaeum, "the region of the body between the anus and the genital organs," early 15c., from Medieval Latin perinaeon, Late Latin perineum, from Greek perinaion, perinaios, in medical writing, "space between the anus and the scrotum," also, in plural, "male genitals," said to be from peri "near" (see peri-) + inan "to empty, carry off by evacuation," a word of unknown origin. Beekes says the notion is "the empty region," which would bring it near to the modern jocular name, taint. Related: Perineal.