Shiva Purana
अनादिमलसंश्लेषः प्रागभावात्स्वभावतः ॥ अत्यंतं परिशुद्धात्मेत्यतो ऽयं शिव उच्यते ॥ २९ ॥
It is by nature a synthesis of the eternal impurities because of its pre-existence. He is called Shiva because he is extremely pure in self.
english translation
anAdimalasaMzleSaH prAgabhAvAtsvabhAvataH ॥ atyaMtaM parizuddhAtmetyato 'yaM ziva ucyate ॥ 29 ॥
hk transliteration by Sanscript