Valmiki Ramayana Ayodhya Kanda Sarga 107 Shloka 12
Original Shlokaपुन्नाम्नो नरकाद् यस्मात् पितरं त्रायते सुतः। तस्मात् पुत्र इति प्रोक्तः पितॄन् यः पाति सर्वतः।।2.107.12।।
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यस्मात् since, सुतः son, पितरम् father, पुन्नाम्नः by name 'Put', नरकात् from hell, त्रायते delivers, तस्मात् for that reason, यः who, सर्वतः in everyway, पितॄन् ancestors, पाति protects, he, पुत्र: इति as putra, प्रोक्तः has been told.
Putra is so called as he delivers his father from a hell known as Put and protects the ancestors in all possible ways.