1. Marked by firm determination or resolution; not shakable
   "a man of unbendable perseverance";
   - firm, steadfast, steady, stiff, unfaltering, unshakable, unwavering, unshakeable
    
2. Resistant to being bent
   "an unbendable iron bar";
   - inflexible

See also: inelastic, muscle-bound, resolute, rigid, semirigid, stiff