Valmiki Ramayana Aranya Kanda Sarga 16 Shloka 35
Original Shlokaभर्ता दशरथो यस्यास्साधुश्च भरतस्सुतः। कथं नु साऽम्बा कैकेयी तादृशी क्रूरशीलिनी।।3.16.35।।
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यस्याः whose, दशरथः Dasaratha, भर्ता husband, साधुः noble, भरतः Bharata, सुतः son, सा she, अम्बा mother, कैकेयी Kaikeyi, तादृशी such a lady, क्रूरशीलिनी a woman of wicked nature, कथं नु how did it happen?
How could mother Kaikeyi, who has a husband like Dasaratha and a noble son like Bharata, be so wicked by nature ?