1650s, euphemism for "underground sewer," from Latin cloaca "public sewer, drain," from cluere "to cleanse," from PIE root *kleue- "to wash, clean" (cf. Greek klyzein "to dash over, wash off, rinse out," klysma "liquid used in a washing;" Lithuanian šluoju "to sweep;" Old English hlutor, Gothic hlutrs, Old High German hlutar, German lauter "pure, clear"). Use in biology, in reference to eliminatory systems of lower animals, is from 1834. Related: Cloacal (1650s); cloacinal (1857).
The body cavity into which the intestinal, urinary, and genital canals empty in birds, reptiles, amphibians, most fish, and monotremes. The cloaca has an opening for expelling its contents from the body, and in females it serves as the depository for sperm. Also called vent