(a.) Not conquerable; indomitable.

assiduous constant continuing diligent dogged enduring faithful immutable impassable impenetrable impregnable inalterable incontestable indefatigable indomitable industrious inexpugnable insistent insuperable insurmountable invincible inviolable invulnerable irresistible lasting loyal never-tiring obstinate overpowering overwhelming patient patient as Job permanent perseverant persevering persistent persisting pertinacious plodding plugging preoccupied proof rapt relentless resistant resistless resolute secure sedulous single-minded sleepless slogging stable steadfast steady stubborn tenacious tight tireless unabating unassailable unbeatable undaunted undefeatable undiscouraged undrooping unfailing unfaltering unflagging unflinching unintermitting uninterrupted unnodding unrelaxing unrelenting unremitting unsleeping unsubduable unsurmountable unswerving untiring unwavering unwearied unwearying unwinking unyielding utterly attentive weariless