Shankar is called as Shoolpaani for holding a trident (Trishul) in his hand to destroy the demoniac forces. The trident, in the hands of Lord Shankar, shows that he does not depend on others for protection of his devotees, rather he directly keeps ready for it. As a mother personally takes care of her children, Shankar also protects his unalloyed devotees. Holding a trident is the proof that how he delivers all beings? He delivers the miscreants by subduing, and saints by assuring. Shankar is the supreme controller, so he keeps materialist and spiritualist, both under his control. Being afraid of punishment, the rouges follow the right path, and being fearful nobles do not reject the right path, and this results a proper order in creation. This Trishul weapon has crushed the evil tendencies of many demons.

This name is stated in Upanishads. A mantra from second section of Mahopanishad greets Shoolpaani as— “ Tryakshah Shoolpaanih Purushah” त्र्यक्षः शूलपाणिः पुरुषः. So this is also a Vedic name. The internal meaning of Shoolpaani points towards Brahmakaar Vritti, the transcendental state of mind. This Brahmakaar Vritti is like a trident, which pierce into the mind and cuts the knots of ignorance. One who possesses such Vritti (mentality), he is called Shoolpaani. Bhaskar Roy quotes a beautiful verse in regard of Shoolpaani—

शिव! तृप्तपुरस्य लिंगमध्ये प्रकटः संस्त्रिगुणात्मकाललोकैः।
शुचिशक्तिदशादिभिस्त्रिशूलं विदधत् त्वं प्रथितोऽसि शूलपाणिः॥

O Lord Shankar! You appeared with a trident in Triptpura, so you are called as Shoolpaani. You are Shoolpaani because you hold a trident which symbolizes the trinity of Guna (qualities of nature), Atma(Spirit), Kaala (time factor), Loka (planets), Agni (fire element), Shakti (potency), Dasha ( condition) etc. Three sharp points of trident of Shoolpani represent many trinities. Goodness, passion and ignorance are three qualities of nature, and these are sustained by Shiva only. Shankar is called Shoolpaani because he manifests himself in three kinds of spirits namely Jiva-atma (individual living entity), Ishwar-atma (controller), and Nirvishesh-atma ( impersonal absolute).
Past, present and future; these three stages of time also depend on Shankar only. Shoolpaani is the prime controller of Heaven, hell and earth planets. Shoolpaani holds the threefold sacrificial fires namely Garhpatya, Aavahniya and Anvaharyapachan. Shoolpaani possesses three potencies namely knowledge, desire and action. Three conditions are also imagined in Shiva only, namely awakening, dreaming and sleeping. Creation, maintenance and annihilation; these three factors also run under his command. Shiva holds three Vedas namely Rika, Sama, Yajur; so he is called as Shoolpaani.

Factually the material world is divided into three fractions namely subject, object and their relativity and Lord Sadashiva upholds this threefold system. Though maintaining everything, the trident becomes the destroyer also at scheduled times. According to Geeta, escape from materialism is possible by a clear understanding of trinity of Kshar(perishable), Akshar (non-perishable) and Uttam-Purusha (Supreme Lord). This work is possible only by mercy of Lord Shoolpaani.

There is a outstanding temple of Shoolpaani at Varanasi. Saur-Purana also states in this regard that after visiting Lord Vishwanath temple one should go to Langlisha, where Brahma and other Gods are serving to Lord Shoolpaani.

अविमुक्तेश्वरं दृष्ट्वा लांगलीशं ततो ययौ।
तत्र ब्रह्मादयो देवाः सेवन्ते शूलपाणिनम्॥

कालारातेः कराग्रे कृतवसतिरुरः शाणशातो रिपूणां
काले काले कुलाद्रिप्रवरतनया कल्पित स्नेहलेपः।
पायान्नः पावकार्चिः प्रसरसखमुखः पापहन्ता नितान्तं
शूल: श्रीपादसेवाभजनरसजुषां पालनैकान्तशीलः॥

Above is a prayer of a devotee which says—the trident, which always stays in the hands of master of death Lord Shankar, which is lovingly worshipped by daughter of mountain Parvati, which is excessively sharpened by the fire of rival’s chest, which eradicates all sins of the worshippers of lotus feet of the Lord, and whose inherent nature is to give protection, may that trident of Lord Shankar save us from all calamities.