fright ‧ ful ‧ ly /ˈfraɪtf ə li/ BrE AmE adverb British English old-fashioned
[ Word Family: adjective : ↑ frightened , ↑ frightening , ↑ frightful ; noun : ↑ fright , ↑ frighteners ; adverb : ↑ frighteningly , ↑ frightfully ; verb : ↑ frighten ]
very:
I’m frightfully sorry about the delay.