A friend or son of a brahmana, when fully qualified, can be called a brahmana and not otherwise. Since Asvatthama's decision is immature, he is purposely called herein the son of a brahmana

Expressions researched:
"A friend or son of a brahmana, when fully qualified, can be called a brahmana and not otherwise. Since Asvatthama's decision is immature, he is purposely called herein the son of a brahmana"

Srimad-Bhagavatam

SB Canto 1

A friend or son of a brahmana, when fully qualified, can be called a brahmana and not otherwise. Since Asvatthama's decision is immature, he is purposely called herein the son of a brahmana.

SB 1.7.19, Translation and Purport

When the son of the brāhmaṇa [Aśvatthāmā] saw that his horses were tired, he considered that there was no alternative for protection outside of his using the ultimate weapon, the brahmāstra [nuclear weapon].

In the ultimate issue only, when there is no alternative, the nuclear weapon called the brahmāstra is applied. The word dvijātmajaḥ is significant here because Aśvatthāmā, although the son of Droṇācārya, was not exactly a qualified brāhmaṇa. The most intelligent man is called a brāhmaṇa, and it is not a hereditary title. Aśvatthāmā was also formerly called the brahma-bandhu, or the friend of a brāhmaṇa. Being a friend of a brāhmaṇa does not mean that one is a brāhmaṇa by qualification. A friend or son of a brāhmaṇa, when fully qualified, can be called a brāhmaṇa and not otherwise. Since Aśvatthāmā's decision is immature, he is purposely called herein the son of a brāhmaṇa.