grim (adj.) Look up grim at Dictionary.com
Old English grimm "fierce, cruel, savage, dire, painful," from Proto-Germanic *grimmaz (cognates: Old Saxon, Old Frisian, Old High German, German grimm, Old Norse grimmr, 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 once it 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, p.p. grummen). Old English also had 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 way to say "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.) Look up grim at Dictionary.com
"spectre, bogey, haunting spirit," 1620s, from grim (adj.).