adverb
Expresses the speaker's strong conjecture or empathetic certainty about another person's feelings or situation. Often used with ~でしょう or ~だろう.
さぞお疲れでしょう。
You must be tired.
さぞかし驚いたことだろう。
I'm sure you were surprised.
Derived from classical Japanese さぞ (然ぞ), a combination of さ (so, that way) and the emphatic particle ぞ. The exact historical development is uncertain, but it has been used as an adverb of conjecture since early modern Japanese.