This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Impossible to capture or enter by force: an impregnable fortress.

adj. Difficult or impossible to attack, challenge, or refute with success: an impregnable argument.

adj. Capable of being impregnated.

adj. Too strong to be penetrated.

adj. Capable of being impregnated.

adj. Not to be stormed, or taken by assault; incapable of being subdued; able to resist attack; unconquerable

adj. Capable of being impregnated, as the egg of an animal, or the ovule of a plant.

Not pregnable; not to be taken or reduced by force: as, an impregnable fortress.

Not to be moved, shaken, or overcome; invincible: as, impregnable virtue.

adj. incapable of being overcome, challenged or refuted

adj. immune to attack; incapable of being tampered with

adj. capable of conceiving

"Given the regular news reports of enterprise data lost through laptop theft, third party misfeasance, WEP hacking, loss of backup tapes, disgruntled employees, etc., it’s not like enterprise data is currently held in impregnable Kryptonite fortresses."

unconquerable    invulnerable    fertile   

unconquerable   

unassailable    impenetrable    inaccessible    insurmountable    invulnerable