Is it ensure, insure, or assure?
c.1600, "kind of lobster," from sea + lion. Later the name of a fabulous animal (in heraldry, etc.), 1660s. Applied from 1690s to various species of large eared seals. As code name for the planned German invasion of Britain, it translates German Seelöwe, announced by Hitler July 1940, scrubbed October 1940.