'terribly' originates from the Latin word 'terribilis,' where 'terrere' meant 'to frighten.'

'terribilis' transformed into the Old French word 'terrible,' and eventually became the modern English word 'terrible,' with 'terribly' as its adverbial form.

Initially, it meant 'causing fear,' but over time it evolved to also mean 'extremely' in a non-fearful context.