Grimmson is a patronymic name of Icelandic origin meaning "son of Grímur," ultimately meaning "fierce", "to thunder," or (in the context of an alternate name for Odin), "mask."
Grímr is one of the names of Norse god Odin, associated with wisdom, healing, death, royalty, the gallows, knowledge, battle, sorcery, poetry, frenzy, and the runic alphabet.
Grimm is the surname of the German linguists Jacob and Wilhelm, better known as the Brothers Grimm, famed authors of various fairy tales known for their darker tones in comparison to other such, or other versions of, stories.
The word "grim" typically has mostly negative connotations (such as the Grim Reaper, a personification of death), being defined as "foreboding or uninviting."
The church grim is a guardian spirit in English and Scandinavian folklore that oversees the welfare of a particular church and protects the churchyard from those who would profane and commit sacrilege against it. It often appears as a black dog but is known to take the form of other animals.