surely; certainly; no doubt; indeed ~
さぞ expresses the speaker's strong assumption or empathy about someone else's feelings or situation, often translated as 'surely' or 'no doubt'.
さぞ is an adverb used when you imagine how someone else feels or what they experienced, emphasizing that it must be to a high degree, based on the situation. It carries a sense of empathy or imagining the other person's state. It is mainly used in conversation or personal writing, not in formal documents.
For example, if a friend tells you they studied all night for an exam, you might say さぞ疲れたでしょう ('you must have been exhausted'). You cannot use it for your own feelings directly. It often appears with だろう or でしょう to show a guess.
Note that さぞ is somewhat formal and can sound a bit literary. In casual speech, you might hear きっと or ぜったい instead, but さぞ adds a nuance of 'I really can imagine it must have been...'.