Is it ensure, insure, or assure?
1570s, from Late Latin hypotenusa, from Greek hypoteinousa "stretching under" (the right angle), fem. present participle of hypoteinein, from hypo- "under" (see sub-) + teinein "to stretch" (see tenet). Formerly often erroneously hypothenuse.
In a right triangle (a triangle that has one right angle), the side opposite the right angle. (See Pythagorean theorem.)
mathematics The side of a right-angled triangle opposite the right angle. (2004-02-15)