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A Brief history and meaning of "Nirmala"
(a) Adjective: Pure, pious, clean, without impurity or without blemish.
(b) Noun: A Sikh sect initiated at the time of Guru Gobind Singh Ji, the Tenth Guru of the Sikhs.
Guru Ji sent five of his chosen Sikhs, which were Ram Singh, Karam Singh, Ganda Singh, Vir Singh and Sobha Singh to Benaras for deeper study of Sanskrit holy books with a view to consolidat the philosophical basis of Sikhism. They returned to Anandpur Sahib after completing the allotted task after 1699, The date of the initiation of the Khalsa.
Guru Ji assigned them a task of translating Sanskrit holy books into Gurmukhi and preaching Gurbani. These six and their followers came to be known a "Nirmalas". They wore salmon colored cloths. They established kuteyas (deras) to preach Gurmat in accordance with Sikh Traditions. Some Nirmalas Saints did not settle at one place and remained moving from place to place preaching Gurmat.
References:
1. Giani Gian Singh. Panth Parkash
2. Bhai Kahan Singh Nabha. Gurshabad Ratnakar Mahan Kosh.
3. Rosemary Goring. (ed) Dictionary of Belifs and Religions.