• śūrāḥ — heroes
  Plural — a class-statement, not a personal jab. Real śūratva (heroism) shows in action; vikatthana (boast) is its counterfeit. The verse is a small dharma-śāstra embedded in the battle narrative.
• vikatthante — boast vainly
  Vi + √kath, intensive — to babble, to brag. Gauḍīya ācāryas reuse this term as a warning against ātma-prakāśana, self-display, in spiritual practice: showing one's bhajana is the demonic counterfeit of doing one's bhajana.
• mumūrṣataḥ — of one about to die
  Desiderative of √mṛ — 'wishing to die.' Genitive of contempt. The Lord identifies Jarāsandha as already psychologically a corpse — his outcome (this birth) is fixed, even if his physical death waits for Bhīma.
• āturasya — of one mentally agitated
  Ātura — afflicted, feverish. The condition of one who cannot rest in any shelter. From the Bhāgavata standpoint, every non-devotee is ātura; the cure is śaraṇāgati.

• Where are you stuck in verbal contest when action would settle the question?
• How can you tell when boasting masks anxiety?

Kṛṣṇa answers boasting with the calm of one who has already decided the outcome. Heroes show their hands, not their tongues — and the words of a man already mentally dead are not worth quoting. Gauḍīyas read this as Bhagavān's instruction on satya-vacana.