Is it ensure, insure, or assure?
c.1300 (mid-13c. in compounds), "artificially banked body of water," variant of pound "enclosed place" (see pound (n.2)). Applied locally to natural pools and small lakes from late 15c. Jocular reference to "the Atlantic Ocean" dates from 1640s. Pond scum (Spirogyra) is from 1864 (also called frog-spittle and brook-silk. As figurative for "someone extremely repulsive," from 1984.