Is it ensure, insure, or assure?
c.1600, from Latin flexionem (nominative flexio) "a bending, swaying; bend, turn, curve," noun of action from past participle stem of flectere "to bend" (see flexible).
flexion flex·ion (flěk'shən)n.
The act of bending a joint or limb in the body by the action of flexors.
The condition of being flexed or bent.