1650s, euphemism for "underground sewer," from Latin cloaca "public sewer, drain," from cluere "to cleanse," from PIE root *kleue- "to wash, clean" (cognates: 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).