Singh is a common name in India used by Rajputs and Sikhs and Hindu martial classes like Dogras, Gurkhas, Gujjars, Jats and Marathas . Singh was first used by Rajputs, then adopted by the Sikh religion and communities like Ahir, Jat etc. also started using it.
Singh is derived from the Sanskrit Simha meaning 'Lion'. It is used throughout Northern India with various transliterations. For example, in Hindi (Devnagari script), the name is written as सिंह (Sinh) and pronounced as सिंघ (Singh). In Punjabi (Gurmukhi script), the name is written as ਸਿੰਘ (Singh) and pronounced as Singh. In Gujarati, it is spelled as Sinh. Marathas use Sing.
Although it is a common misconception to call both the lion and the Tiger by the common name "Sher" (of Turko-Persian origin) in Northern India and Pakistan, most terms for the tiger in various Indo-Aryan tongues include Baagh (Hindi), Baaghaa (Bengali), Baghiyaad (Punjabi) and Vaagh (Gujarati and Marathi). All these are derived from the Sanskrit word for tiger, Vyaagraha.
"Singh" was originally used by the ancient Hindu martial classes of Northern India such as the Rajputs.Reference: Dr. Gopal Singh, History of the Sikhs After the birth of the Khalsa brotherhood in 1699, the Sikhs adopted the name "Singh" due to the wishes of Guru Gobind Singh. The Rajput martial way of life was a great influence upon the martial traditions of the people of the Punjab and Shiwalik hill ranges, as cited in Guru Gobind Singh's monumental work and partial autobiography, Bichitra Natak. "Kaur" was the equivalent name for women, since Rajput females used "Kumari".Dr. McCleod, Head of Sikh Studies, Department of South Asian Studies, McMaster University, Hamilton, Ontario, Canada
Guru Gobind Singh, gave the name "Singh" to all the Sikh men so that all in the community would be equals. Women had not been treated as equals before the time of the Gurus, so to ensure equality, the name Kaur (meaning "Princess") was given to all the Sikh women. The Guru said, "You are my beloved princesses, my daughters. You must be respected. How can this world be without you?" These two names were given to the Sikhs on April 13 and 14, 1699.
Among Sikhs and Hindus:
An important distinction to be noted here is that "while all Sikhs are Singhs, all Singhs are not Sikhs."Khushwant Singh, A History of the Sikhs, Volume I.