Source : Webster's Revised Unabridged Dictionary (1913)

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.