Valmiki Ramayana Ayodhya Kanda Sarga 110 Shloka 23

सपत्नया by her cowife, गर्भजिघांसया with a motive to destroy the embryo, तस्यै to her, गरः poison, दत्तः given, तेन for that reason, गरेण सहैव with poison, जातः born, सः he, सगरः Sagara, अभवत् became.

With a motive to destroy the embryo her cowife had poisoned her. That child was born with gara or poison for which he was called Sagara.