
Old Norse word grimmr comes from Proto-Germanic *grimmaną (To rage, roar, be angry.), Proto-Indo-European *gʰrem-
*grimmaną (Proto-Germanic)
To rage, roar, be angry.
*gʰrem- (Proto-Indo-European)
*grimmaz (Proto-Germanic)
Grim, grimly, fierce.
grimmr (Old Norse)