grimyoudaoicibaDictYouDict[grim 词源字典]
grim: [OE] Indo-European *ghrem-, *ghromprobably originated in imitation of the sound of rumbling (amongst its descendants was grumins ‘thunder’ in the extinct Baltic language Old Prussian). In Germanic it became *grem-, *gram-, *grum-, which not only produced the adjective *grimmaz (source of German grimm, Swedish grym, and English, Dutch, and Danish grim) and the English verb grumble [16], but was adopted into Spanish as grima ‘fright’, which eventually arrived in English as grimace [17].
=> grimace, grumble[grim etymology, grim origin, 英语词源]
grim (adj.)youdaoicibaDictYouDict
Old English grimm "fierce, cruel, savage; severe, dire, painful," from Proto-Germanic *grimmaz (cognates: Old Saxon, Old Frisian, Old High German, German grimm "grim, angry, fierce," Old Norse grimmr "stern, horrible, dire," Swedish grym "fierce, furious"), from PIE *ghrem- "angry," 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.)youdaoicibaDictYouDict
"spectre, bogey, haunting spirit," 1620s, from grim (adj.).