It could be a form of Selima, which in turn is a name first recorded in a poem by Thomas Gray (died 1771).
One possibility is that Selima was influenced by the Arabic name Selim meaning "peaceful".
[2] The use of Selma in Germany and Scandinavia stems from the Ossianic poetry of James Macpherson (died 1796), where it appears as a place name.
[1][2][3][4] Its specific popularity in Sweden is likely due to the Selma poems of Frans Michael Franzén (died 1847).
It was later introduced into Denmark by Swedish immigrants, after which it likely became more common due to the works of the author Selma Lagerlöf (died 1940).