1. indispensable essential
  2. indecipherable not easily decoded
  3. insuperable incapable of being surpassed or excelled
  4. inseparable not capable of being split
  5. inexorable impossible to prevent, resist, or stop
  6. undecipherable not easily deciphered
  7. indescribable defying expression
  8. interoperable able to exchange and use information
  9. indisputable not open to question; obviously true
  10. vulnerable capable of being wounded or hurt
  11. undesirable not wanted
  12. unbearable incapable of being put up with
  13. invulnerable immune to attack; impregnable
  14. undiscoverable not able to be ascertained; resisting discovery
  15. undecipherably in an illegible manner
  16. inseparably without possibility of separation
  17. insufferable extremely unpleasant or annoying
  18. insuperably to an insuperable degree
  19. indubitable too obvious to be doubted
  20. indescribably to an inexpressible degree