さぞ (さぞや。さぞかし)
さぞ (さぞや。さぞかし) · sazo (sazoya。sazokashi) · JLPT N1 · noun
certainly
さぞ = さ (so, that way) + ぞ (emphatic): surely it is so, no doubt
Etymology: さ means 'so' (as in そう) and ぞ is an emphatic sentence-end particle, so さぞ says 'it must surely be so'. Example: さぞ疲れたでしょう means 'you must surely be tired'.