Pronunciation: (grim),[key] —adj.,grim•mer, grim•mest. 1. stern and admitting of no appeasement or compromise: grim determination; grim necessity. 2. of a sinister or ghastly character; repellent: a grim joke. 3. having a harsh, surly, forbidding, or morbid air: a grim man but a just one; a grim countenance. 4. fierce, savage, or cruel: War is a grim business.