Not capable of being conquered or vanquished or overcome.

Roget 159: strong, mighty, vigorous, forcible, hard, adamantine, stout, robust, sturdy, hardy, powerful, potent, puissant, valid.    resistless, irresistible, invincible, proof against, impregnable, unconquerable, indomitable, dominating, inextinguishable, unquenchable; incontestable; more than a match for; overpowering, overwhelming; all powerful, all sufficient; sovereign.    able-bodied; athletic; Herculean, Cyclopean, Atlantean^†; muscular, brawny, wiry, well-knit, broad-shouldered, sinewy, strapping, stalwart, gigantic.    manly, man-like, manful; masculine, male, virile.    unweakened^†, unallayed, unwithered^†, unshaken, unworn, unexhausted^†; in full force, in full swing; in the plenitude of power.    stubborn, thick-ribbed, made of iron, deep-rooted; strong as a lion, strong as a horse, strong as an ox, strong as brandy; sound as a roach; in fine feather, in high feather; built like a brick shithouse; like a giant refreshed.    ... show more

Roget 604a: persevering, constant; steady, steadfast; undeviating, unwavering, unfaltering, unswerving, unflinching, unsleeping^†, unflagging, undrooping^†; steady as time; unrelenting, unintermitting^†, unremitting; plodding; industrious etc. 682; strenuous etc. 686; pertinacious; persisting, persistent.    solid, sturdy, staunch, stanch, true to oneself; unchangeable etc. 150; unconquerable etc. (strong) 159; indomitable, game to the last, indefatigable, untiring, unwearied, never tiring.    ... show more

Roget 719: resisting etc. v.; resistive, resistant; refractory etc. (disobedient) 742; recalcitrant, renitent; up in arms.    repulsive, repellant.    proof against; unconquerable etc. (strong) 159; stubborn, unconquered; indomitable etc. (persevering) 604.1; unyielding etc. (obstinate) 606.    ... show more

Incapable of being surmounted or excelled.

synonym: insuperable.