'grimly' originates from Old English, specifically the word 'grimlic,' where 'grim' meant 'fierce' and '-lic' was an adverbial suffix.

'grimlic' transformed into the Middle English word 'grimliche,' and eventually became the modern English word 'grimly'.

Initially, it meant 'fiercely' or 'sternly,' and this meaning has largely remained the same in modern usage.