ex ‧ treme ‧ ly S2 W2 /ɪkˈstriːmli/ BrE AmE adverb [+ adjective/adverb]
[ Word Family: noun : ↑ extreme , ↑ extremism , ↑ extremist , ↑ extremity ; adjective : ↑ extreme , ↑ extremist ; adverb : ↑ extremely ]
to a very great degree:
Earthquakes are extremely difficult to predict.
I’m extremely sorry to have troubled you.