An adverb expressing strong supposition or sympathy (e.g., 'surely' or 'no doubt'); often appears in phrases like 'さぞ〜だろう/でしょう'.
長旅で疲れたでしょう。さぞお疲れのことだ。
You've had a long trip and must be tired. You must be exhausted.
彼は試験に落ちたと聞いて、さぞがっかりしただろう。
Hearing that he failed the exam, he must have been very disappointed.
ご家族を亡くされたと聞き、さぞお悲しみのことでしょう。
Having heard you lost a family member, you must be very saddened.