Canto 2 -
Ayodhyā-kāṇḍa
Chapter 107: Rāma Explains why the King Gave the Kingdom to Kaikeyī
Text 2.107.12

पुन्नाम्नो नरकाद्यस्मात्पितरं त्रायते सुतः।
तस्मात्पुत्र इति प्रोक्तः पितॄन्यत्पाति वा सुतः॥

pun-nāmno narakād yasmāt pitaraṁ trāyate sutaḥ
tasmāt putra iti proktaḥ pitṝn yat pāti vā sutaḥ

put-nāmnaḥ = named Put; narakāt = from the hell; yasmāt = because; pitaram = his father; trāyate = protects; sutaḥ = he; tasmāt putraḥ iti proktaḥ = is called a putra; pitṝn = his forefathers; yat = because; pāti = he protects; = or; sutaḥ = a son.

A son is called a putra because he protects his father from the hell named Put or because he protects his forefathers.1

A son protects his forefathers, that is, he has them sent to Svarga by performing iṣṭa and pūrta pious activities for the sake of his forefathers.1

1 Technical note: vyutpatty-antaram āha pitṝn iti. pitṝn pāti ity arthe putrasya pṛṣodarāditvāt sādhutvam. pitṝn uddiśya kṛteṣṭā-pūrtādinā svar-loka-prāpaṇena teṣāṁ rakṣaṇam ity āhuḥ.

 1 See Rāmāyaṇa 1.21.8 commentary about iṣṭa and pūrta pious activities.