1. used as intensifiers

- terribly interesting

Similar word(s): awful, awfully, frightfully

2. in a terrible manner

- she sings terribly

Similar word(s): abominably, abysmally, atrociously, awfully, rottenly

Sentences with terribly as an adverb:

- The lion roared terribly.

- He's terribly busy and you really shouldn't bother him.

- She even signed up for karaoke, although she sang terribly.