Stern

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