hypotenuse (n.)

the side of a right triangle that is opposite the right angle, 1570s, from Late Latin hypotenusa, from Greek hypoteinousa "stretching under" (the right angle), fem. present participle of hypoteinein, from hypo- "under" (see hypo-) + teinein "to stretch," from PIE root *ten- "to stretch." Formerly often erroneously hypothenuse. Related: Hypotenusal.