From OE 'grimm' (fierce) — in Norse mythology, 'Grímr' was Odin's name; the Brothers Grimm bear a surname meaning 'fierce.'
Very serious or forbidding in manner or appearance; harsh, merciless, or depressing.
From Old English grimm (fierce, cruel, savage, dire), from Proto-Germanic *grimmaz (fierce, angry, grim), from PIE *gʰrem- (to be angry, to make a harsh noise, to thunder). The Proto-Indo-European root *gʰrem- combined anger with harsh sound — to thunder, to roar — and this dual sense of ferocity and noise pervades its descendants. Old English grimm described both fierce warriors and
The Brothers Grimm — Jacob and Wilhelm — bear a surname meaning 'fierce' or 'severe,' making 'Grimm's Fairy Tales' etymologically 'Fierce Tales.' Odin was called 'Grímr' (the masked, fierce one) in Norse mythology, and the adjective appears in Beowulf to describe monsters and battlefields.