This is the 7576th most frequent German word.
grimm
"Grimm" in German means "wrath" or "fury."
In this sentence, 'Grimm' refers to the surname of the famous German folklorists, and so it identifies them in the context of literature.
Die Märchen der Brüder Grimm sind weltweit bekannt.
The tales of the Brothers Grimm are famous worldwide.
Here, 'grimmigen' is the adjective form of 'grimm', meaning 'grim' or 'stern,' describing someone's expression.
Er schaute mit einem grimmigen Blick.
He stared with a grim look.
In this sentence, 'grimm' is used as an adjective meaning 'stern,' and indicates someone's mood or behavior.