The king of the celestials himself, incapable of defeating you in battle. The son is one's own self. It is for this that you have been defeated by him.
न हि त्वां देवराजोऽपि समरेषु पराजयेत्। आत्मा पुत्रः स्मृतस्तस्मात् तेनेहासि पराजितः॥