Amitrajit is one of the thousand and eight names of God Śiva (Siva). Amitra means enemy. Mahādeva (Mahadeva) destroys the tyrannical Asuras, enemies of gods. So he is Amitrajit. He has also overcome the invisible enemies like the evil passions of lust, anger etc. He easily wins over all enemies — both external and internal, and so he is known as Amitrajit —
amitrajit vāhyābhyantaśatrujit (Nīlakaṇṭha).