Grim
Old English, of Germanic origin; related to Dutch grim and German grimm .
wiktionary
From Middle English grim, from Old English grim, grimm, from Proto-West Germanic *grimm, from Proto-Germanic *grimmaz, from Proto-Indo-European *gʰrem-(“to resound, thunder, grumble, roar”). Noun sense derives from adjective, from 1620s.
From Middle English grim, grym, greme, from Old English *grimu, *grimmu, from Proto-Germanic *grimmį̄(“anger, wrath”), from Proto-Indo-European *gʰrem-(“to resound, thunder, grumble, roar”). Cognate with Middle Dutch grimme, Middle High German grimmef(“anger”), modern German Grimmm.
etymonline
grim (adj.)
Old English grimm "fierce, cruel, savage; severe, dire, painful," from Proto-Germanic *grimma- (source also of Old Saxon, Old Frisian, Old High German, German grimm "grim, angry, fierce," Old Norse grimmr "stern, horrible, dire," Swedish grym "fierce, furious"), from PIE *ghremno- "angry," which is perhaps imitative of the sound of rumbling thunder (compare Greek khremizein "to neigh," Old Church Slavonic vuzgrimeti "to thunder," Russian gremet' "thunder").
A weaker word now than it once was; sense of "dreary, gloomy" first recorded late 12c. It also had a verb form in Old English, grimman (class III strong verb; past tense gramm, past participle grummen), and a noun, grima "goblin, specter," perhaps also a proper name or attribute-name of a god, hence its appearance as an element in place names.
Grim reaper as a figurative phrase for "death" is attested by 1847 (the association of grim and death goes back at least to 17c.). A Middle English expression for "have recourse to harsh measures" was to wend the grim tooth (early 13c.).
grim (n.)
"spectre, bogey, haunting spirit," 1620s, from grim (adj.).
