Because he (Arjuna) accepted Krsna as spiritual master, so spiritual master has the right to chide his disciple. So He immediately chided him

Expressions researched:
"spiritual master has the right to chide his disciple"

General Lectures

Because he accepted Kṛṣṇa as spiritual master, so spiritual master has the right to chide his disciple. So He immediately chided him, that aśocyān anvaśocas tvaṁ prajñā-vādāṁś ca bhāṣase. "My dear Arjuna, you are talking just like a very learned man, but you are fool number one." That was His first words.

Lecture -- Montreal, June 26, 1968:

So this Kṛṣṇa consciousness movement is to cure the disease of the soul. Disease of the soul. Every soul, every person, is mistaken to accept this body as his self or the mind as his self. This is the difference. Yasyātma-buddhiḥ kuṇape tri-dhātuke, sa eva go-kharaḥ (SB 10.84.13). Anyone who is accepting this body as self, he's either an ass or a cow. Misconception.

So people are not interested. We are trying, the Bhagavad-gītā is trying. In the beginning, beginning of the instruction . . . just like Arjuna was identifying himself with this body in the beginning, and he was thinking that "He is my grandfather, he is my son, he is my nephew, he is my . . ." so on, so on. Kṛṣṇa replied him. When Arjuna was unable to make a solution of his problems, he surrendered to Kṛṣṇa and said, śiṣyas te 'haṁ śādhi māṁ prapannam (BG 2.7): "I surrender unto You as Your disciple. Please save me from this perplextion."

Then Kṛṣṇa in the first instance replied, aśocyān anvaśocas tvaṁ prajṣā-vādāṁś ca bhāṣase (BG 2.11): "My dear Arjuna . . ." Because he accepted Kṛṣṇa as spiritual master, so spiritual master has the right to chide his disciple. So He immediately chided him, that aśocyān anvaśocas tvaṁ prajṣā-vādāṁś ca bhāṣase: "My dear Arjuna, you are talking just like a very learned man, but you are fool number one." That was His first words. Of course, He did not say directly "fool number one," but He said indirectly, that "No learned man speaks like this, as you are speaking." That means, "You are not learned man. You are fool."

He indirectly said, nānuśocanti paṇḍitāḥ. Paṇḍita means learned. "No learned man speaks like that. But because you are speaking like that, that means you are not learned man. Or in one word, you are fool." Because Kṛṣṇa . . . before Kṛṣṇa, he was speaking that, "If I kill my family members, then such-and-such thing will happen. Then the women will become polluted and there will be unwanted children, varṇa-saṅkara. And as soon as there is varṇa-saṅkara, full of unwanted children, this world will be hell." These are facts. These are facts. The world has become hell due to unwanted children. That is the fact.

So Kṛṣṇa was speaking . . . Arjuna was speaking just like ordinary gentleman on the material field, but Kṛṣṇa, when He took up his charge, He said that, "You are hovering over the material plane. That is not your learning. The learning is when you understand from the spiritual platform."