Is it ensure, insure, or assure?
early 15c., "a knot or lump," from Latin nodus "knot" (see net (n.)). Originally borrowed c.1400 in Latin form, meaning "lump in the flesh." Meaning "point of intersection" (originally of planetary orbits with the ecliptic) first recorded 1660s.
A knob, knot, protuberance, or swelling.
A protuberant growth or swelling in a tissue.
A knuckle or finger joint.