Grimly

[grim] /grɪm/

adjective, grimmer, grimmest.
1.
stern and admitting of no appeasement or compromise:
grim determination; grim necessity.
2.
of a sinister or ghastly character; repellent:
a grim joke.
3.
having a harsh, surly, forbidding, or morbid air:
a grim man but a just one; a grim countenance.
4.
fierce, savage, or cruel:
War is a grim business.
/ɡrɪm/
adjective grimmer, grimmest
1.
stern; resolute: grim determination
2.
harsh or formidable in manner or appearance
3.
harshly ironic or sinister: grim laughter
4.
cruel, severe, or ghastly: a grim accident
5.
(archaic or poetic) fierce: a grim warrior
6.
(informal) unpleasant; disagreeable
7.
hold on like grim death, to hold very firmly or resolutely

Old English grimlic (adj.) “fierce, bloodthirsty, cruel,” grimlice (adv.); see grim + -ly. Cf. Middle Dutch grimmelijc, Old Norse grimmligr.
adj.

Old English grimm “fierce, cruel, savage, dire, painful,” from Proto-Germanic *grimmaz (cf. 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 (cf. 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.).
n.

“spectre, bogey, haunting spirit,” 1620s, from grim (adj.).