1. (a.) Exact; strict; severe; relentless; uncompromising\; manifesting, exercising, or favoring rigor; allowing no abatement or mitigation; scrupulously accurate; as, a rigorous officer of justice; a rigorous execution of law; a rigorous definition or demonstration.

2. (a.) Severe; intense; inclement; as, a rigorous winter.

3. (a.) Violent.

Herculean Siberian abstruse accurate acid acidulous acrid acrimonious acute adamant adamantine adverse algid antagonistic arctic arduous ascetic astringent attentive below zero biting bitter bitterly cold bleak boreal brisk brumal brutal burdensome cast-iron caustic close cold cold as charity cold as death cold as ice cold as marble complex conflicting conscientious constant contrary correct counter counteractive crisp critical cutting delicate demanding detailed detrimental difficile difficult direct double-edged dour draconian drastic edged escharotic even exact exacting excessive exigent exorbitant express exquisite extravagant extreme faithful fierce fine finical finicking finicky firm flinty formidable freezing freezing cold frigid fundamentalist furious fussy gelid glacial great grim hairy hard hard-core hard-earned hard-fought harmful harsh hibernal hidebound hiemal hostile hyperborean ice-cold ice-encrusted icelike icy immoderate immovable immutable implacable impliable in opposition incisive inclement inelastic inerrable inerrant inexorable infallible inflexible inimical intemperate intense intransigent intricate iron ironbound ironclad ironhanded irreconcilable jawbreaking keen knotted knotty laborious mathematical mean meticulous micrometrically precise microscopic minute miserable mordacious mordant muscle-bound narrow nice nipping nippy no picnic not easy numbing obdurate obstinate onerous operose opposed opposing opposite oppressive orthodox outrageous particular penetrating piercing pinching pinpoint poignant precise precisianistic precisionistic procrustean proper punctilious punctual purist puristic puritan puritanic raw refined relentless religious religiously exact right rigid rigorist rigoristic rock-ribbed rockbound rough rugged scathing scientific scientifically exact scrupulous scrutinizing set with thorns severe sharp sinister sleety slushy snappy spiny splitting square stabbing steely steep stern stiff stinging stone-cold straightlaced straitlaced strenuous stressful strict strident stringent stubborn subtle subzero supercooled tart thorny ticklish toilsome tough trenchant tricky troublesome troublous trying unaffected unalterable unbending unchangeable uncompromising unconscionable undeviating unerring unfavorable ungiving unmoved unrelenting untoward unyielding uphill vehement venomous violent virulent