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]

The Collaborative International Dictionary of English v.0.48:

Flex \Flex\, n.
   Flax. [Obs.] --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flex
    n 1: the act of flexing; "he gave his biceps a flex to impress
         the ladies"
    v 1: contract; "flex a muscle"
    2: exhibit the strength of; "The victorious army flexes its
       invincibility"
    3: form a curve; "The stick does not bend" [syn: bend, flex]
       [ant: straighten, unbend]
    4: bend a joint; "flex your wrists"; "bend your knees" [syn:
       flex, bend]
    5: cause (a plastic object) to assume a crooked or angular form;
       "bend the rod"; "twist the dough into a braid"; "the strong
       man could turn an iron bar" [syn: flex, bend, deform,
       twist, turn] [ant: unbend]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "flex":
   S-curve, arch, bend, bend back, bending, bow, bowing, cable,
   conflexure, contract, cord, crook, curl, curve, decurve, deflect,
   deflection, dome, embow, exercise, extension, flection, flexure,
   geanticline, geosyncline, give, hairpin turn, hook, hump, hunch,
   incurvate, incurve, inflect, inflection, lead, loop, meander,
   oxbow, recurve, reflect, reflection, reflex, retroflex, round, sag,
   stretch, swag, sweep, tense, tighten, turn, turning, vault, wind,
   wire



The Free On-line Dictionary of Computing (30 December 2018):

FLEX

    1. Faster LEX.

   2. A real-time language for dynamic environments.

   ["FLEX: Towards Flexible Real-Time Programs", K. Lin et al,
   Computer Langs 16(1):65-79, Jan 1991].

   3. An early object-oriented language developed for the
   FLEX machine by Alan Kay in about 1967.  The FLEX language
   was a simplification of Simula and a predecessor of
   Smalltalk.

   (1995-03-29)


The Free On-line Dictionary of Computing (30 December 2018):

Flex

    A system developed by Ian Currie (Iain?)
   at the (then) Royal Signals and Radar Establishment at
   Malvern in the late 1970s.  The hardware was custom and
   microprogrammable, with an operating system, (modular)
   compiler, editor, garbage collector and filing system
   all written in Algol-68.  Flex was also re-implemented on
   the Perq(?).

   [I. F. Currie and others, "Flex Firmware", Technical Report,
   RSRE, Number 81009, 1981].

   [I. F. Currie, "In Praise of Procedures", RSRE, 1982].

   (1997-11-17)