More grim; comparative form of grim, meaning more serious, forbidding, or harsh in appearance.
Grim + -er (comparative suffix). Standard English comparative formation from grim (Old Norse/Germanic: fierce, stern).
Grimmer is one of those words where comparative and superlative forms are irregular in history—Old English speakers would have said 'more grim,' but our Germanic roots gave us these -er/-est shortcuts.
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