किमिदं दैवयोगेन भूतं लोकविमोहनम् ॥ ३३ ॥
vajraḥ pratihato yataḥ
kim idaṁ daiva-yogena
tasmāt — therefore; indraḥ — the King of heaven; abibhet — became very fearful; śatroḥ — from the enemy (Namuci); vajraḥ — the thunderbolt; pratihataḥ — was unable to hit and returned; yataḥ — because; kim idam — what is this; daiva-yogena — by some superior force; bhūtam — it has happened; loka-vimohanam — so wonderful to the people in general.
When Indra saw the thunderbolt return from the enemy, he was very much afraid. He began to wonder whether this had happened because of some miraculous superior power.
Indra’s thunderbolt is invincible, and therefore when Indra saw that it had returned without doing any injury to Namuci, he was certainly very much afraid.