Not capable of being surmounted or overcome.

synonym: insurmountable.

Roget 471: impossible; not possible etc. 470; absurd, contrary to reason; unlikely; unreasonable etc. 477; incredible etc. 485; beyond the bounds of reason, beyond the bounds of possibility, beyond the realm of possibility; from which reason recoils; visionary; inconceivable etc. (improbable) 473; prodigious etc. (wonderful) 870; unimaginable, inimaginable^†; unthinkable.    impracticable unachievable; unfeasible, infeasible; insuperable; unsurmountable^†, insurmountable; unattainable, unobtainable; out of reach, out of the question; not to be had, not to be thought of; beyond control; desperate etc. (hopeless) 859; incompatible etc. 24; inaccessible, uncomeatable^†, impassable, impervious, innavigable^†, inextricable; self-contradictory.    out of one's power, beyond one's power, beyond one's depth, beyond one's reach, beyond one's grasp; too much for; ultra crepidam [Lat.].    ... show more

Incapable of being surmounted or climbed.

synonym: unclimbable.