The Collaborative International Dictionary of English v.0.48:

Flexible \Flex"i*ble\, a. [L. flexibilis: cf. F. flexible.]
   1. Capable of being flexed or bent; admitting of being
      turned, bowed, or twisted, without breaking; pliable;
      yielding to pressure; not stiff or brittle.
      [1913 Webster]

            When the splitting wind
            Makes flexible the knees of knotted oaks. --Shak.
      [1913 Webster]

   2. Willing or ready to yield to the influence of others; not
      invincibly rigid or obstinate; tractable; manageable;
      ductile; easy and compliant; wavering.
      [1913 Webster]

            Phocion was a man of great severity, and no ways
            flexible to the will of the people.   --Bacon.
      [1913 Webster]

            Women are soft, mild, pitiful, and flexible. --Shak.
      [1913 Webster]

   3. Capable or being adapted or molded; plastic,; as, a
      flexible language.
      [1913 Webster]

            This was a principle more flexible to their purpose.
                                                  --Rogers.

   Syn: Pliant; pliable; supple; tractable; manageable; ductile;
        obsequious; inconstant; wavering. -- Flex"i*ble*ness,
        n. -- Flex"i*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

flexible
    adj 1: capable of being changed; "flexible schedules" [ant:
           inflexible]
    2: able to flex; able to bend easily; "slim flexible birches"
       [syn: flexible, flexile] [ant: inflexible]
    3: able to adjust readily to different conditions; "an adaptable
       person"; "a flexible personality"; "an elastic clause in a
       contract" [syn: elastic, flexible, pliable, pliant]
    4: bending and snapping back readily without breaking [syn:
       flexible, whippy]
    5: making or willing to make concessions; "loneliness tore
       through him...whenever he thought of...even the compromising
       Louis du Tillet" [syn: compromising, conciliatory,
       flexible] [ant: inflexible, sturdy, uncompromising]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "flexible":
   able to adapt, acquiescent, adaptable, adaptive, adjustable,
   all-around, alterable, alterative, ambidextrous, amenable,
   amphibious, bendable, bending, biddable, bouncy, buoyant,
   changeable, checkered, complaisant, compliant, conformable,
   convenient, cooperative, creased, crimped, docile, dog-eared,
   doubled, ductile, easy, elastic, ever-changing, extensible,
   extensile, fabricable, facile, feasible, fictile, flexile,
   flexuous, flounced, fluid, fluted, foldable, folded, folding,
   foolproof, formable, formative, gathered, generally capable,
   giving, handy, impermanent, impressible, impressionable,
   kaleidoscopic, like putty, limber, lissome, lithe, lithesome,
   lively, malleable, manageable, maneuverable, many-sided,
   metamorphic, mobile, modifiable, moldable, movable, mutable,
   nonuniform, obedient, other-directed, permutable, persuadable,
   persuasible, plaited, plastic, pleated, pliable, pliant, plicate,
   plicated, plicatile, practical, protean, proteiform, quilled,
   receptive, resilient, resourceful, responsive, rubbery, ruffled,
   sensitive, sequacious, shapable, springy, stretch, stretchable,
   stretchy, submissive, supple, susceptible, tensile, tractable,
   tractile, transient, transitory, tucked, twilled, two-handed,
   unaustere, undemanding, unexacting, unharsh, unsevere, unstrict,
   untroublesome, variable, versatile, whippy, wieldable, wieldy,
   willowy, yielding