Dictionary entry

Flex

Webster's Dictionary 1913

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.