Impossible to resist; overpowering:
— Irresistible (or resistless) impulses.

synonym: irresistible.

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 601: necessary, needful etc. (requisite) 630.    fated; destined etc. v.; elect; spellbound compulsory etc. (compel) 744; uncontrollable, inevitable, unavoidable, irresistible, irrevocable, inexorable; avoidless^†, resistless.    involuntary, instinctive, automatic, blind, mechanical; unconscious, unwitting, unthinking; unintentional etc. (undesigned) 621; impulsive etc. 612.    ... show more

Offering no resistance:
— Resistless hostages.

synonym: unresisting.