The Collaborative International Dictionary of English v.0.48:

Flex \Flex\, v. t. [imp. & p. p. Flexed; p. pr. & vb. n.
   Flexing.] [L. flexus, p. p. of flectere to bend, perh.
   flectere and akin to falx sickle, E. falchion. Cf. Flinch.]
   To bend; as, to flex the arm.
   [1913 Webster]