'grossly' is formed in English from the adjective 'gross' + the adverbial suffix '-ly'. 'gross' ultimately comes from Old French 'gros', from Late Latin 'grossus' meaning 'thick, coarse'.

'gross' entered Middle English from Old French 'gros' and later took on senses of 'large' or 'whole' in English; the adverb 'grossly' developed by adding '-ly' to express 'in a gross manner' or 'to a great degree'.

Initially 'gross' meant 'thick' or 'coarse' in Late Latin and Old French; over time it shifted to mean 'large in amount' or 'blatantly obvious', and 'grossly' came to mean 'to a great or conspicuous degree'.