पर्वणि on the day of the full Moon and new Moon, समुद्र इव like sea, वीर्येण with energy, विवर्धमान: growing, ऋषिसत्तमम् foremost of ascetics, वसिष्ठम् Vasishta, तदा then, हतमेव slain only, मेने thought over.

Like the sea during the full moon, with his increased power (blessed with the invincible strength of the weapons) Viswamitra thought the foremost of ascetics Vasishta was (already) slain. (It shows the height of the king's ignorance and arrogance).