From Middle Persian [Book Pahlavi needed] (swlʾh/sūrāx/) from earlier form [Book Pahlavi needed] (swlʾk'/sūrāg/). Ultimately from Proto-Indo-European*ḱouH-r-o- and suffix ـاک(/-āk/)
^ Miller, Corey, Livingston, Jace, Vinson, Mark, Triebwasser Prado, Thomas (2014) Persian Dialects: As Spoken in Iran[1], University of Maryland Center for Advanced Study of Language, page 14: “Peisikov (1960, section 25) notes several words with /r/ in the standard language that can be pronounced with /l/ in Tehrani: دیوار 'wall' /dival/, برق 'electricity' /bälɣ/, سوراخ 'hole' /sulax/.”