IPA: /ɡrɪməst/
KK: /grɪmɪst/
The most serious, harsh, or gloomy in appearance or attitude.
The news about the storm was the grimmest I had ever heard.
Comparative: grimmer
Superlative: grimmest
Grimmest is formed from "grim" (from Old English "grim", meaning fierce or cruel) and the suffix "-est" (meaning the most). The word "grimmest" refers to the most fierce or cruel state of something.
Think of the word 'grim' meaning fierce or cruel, and remember that 'grimmest' means the most fierce or cruel.