Synonyms for Rigorous:

adj.

all (adjective)
strict.
economical (adjective)
spartan.
effortful (adjective)
effortful.
fastidious (adjective)
flawless, persnickety, severe, particular, delicate, careful, diligent, choosy, prim, literal, fussy, prudish, correct, discerning, hypercritical, critical, priggish, mincing, scrupulous, Qualmish, picky, precise, demanding, discriminating, punctilious, methodical, conscientious, dainty, perspicacious, epicurean, exacting, proper, fastidious, squeamish, accurate, finicky, meticulous.
precise (adjective)
right, correct, definite, exact, meticulous, strict, precise, accurate, proper.
severe (adjective)
correct, intense, uncompromising, acute, cutting, ascetic, brusque, fundamental, draconian, demanding, austere, sharp, authoritarian, rigid, bleak, disciplined, censorious, relentless, exacting, grim, spartan, strait-laced, obdurate, dry, chilly, imperial, acerbic, unbending, precise, cool, spare, inflexible, dour, brisk, oppressive, strict, stiff-necked, abrupt, basic, blunt, lean, keen, hidebound, astringent, gruff, caustic, meticulous, short, severe, stringent, stark, tart, stern, intolerant, frosty, crisp, acrimonious, icy, piquant, obstinate, raw, harsh, puritanical, critical, curt, prudish.
severe; exact (adjective)
definite, dogmatic, austere, precise, accurate, punctilious, exacting, brutal, rigid, scrupulous, ascetic, meticulous, proper, harsh, hard, stringent, uncompromising, right, inflexible, burdensome, onerous, strict, oppressive, stern, correct.
veridical (adjective)
veridical.

n.

awful, desperate, extreme, grave, profound, serious, dire. cautious, organized, provident, systematic, thorough, usual, painstaking. faithful, full. errorless, true. honest, veracious, veridical, real. exact (noun)
dogmatic.
rigorous (noun)
tight, strict, cruel, stringent, harsh, unkind, exact, brutal, demanding, hard.

Other synonyms:

errorless, effortful, backbreaking, difficult, desperate. trying, profound, veridical, dire, laborious, burdensome. veracious, grave, weighty, exigent, extreme, arduous. awful, taxing, true, faithful. formidable, tough. full. heavy. oppressive
onerous.

Usage examples for rigorous

  1. You see me here in the very prime of my youth, cut off like an untimely flower in the rigorous season, through my having been too much addicted to a voluptuous and irregular course of life, which has been the occasion of my committing those crimes for which I am now to suffer. – Lives Of The Most Remarkable Criminals Who have been Condemned and Executed for Murder, the Highway, Housebreaking, Street Robberies, Coining or other offences by Arthur L. Hayward
  2. That is why the game laws are so rigorous – Tales of Fantasy and Fact by Brander Matthews