Stern

hard severe strict tough inflexible rigid grim austere relentless harsh implacable unyielding inexorable unsparing hardheaded stubborn hard-nosed steely forbidding authoritarian cruel hard-line rigorous flinty ascetic astringent disciplinary frowning hard-boiled hard-core mulish rough stiff-necked bitter by the book bullheaded dyed-in-the-wool hang-tough hard-shell mortified