From The Collaborative International Dictionary of English v.0.48 [gcide]: 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]