In some part of the world certain people often refer to some type of (blunts, joints) as Ls. The reasons vary and some people are not even aware of the 'L' slang. Some people reference blunts as Ls either because when a blunt is done getting rolled up it resembles a lowercase 'l', while some people call them Ls because of the "EL Producto"(a blunt manufacturing company).
Some individuals claim L's is when a joint is rolled up with two papers and are connected to have an 'L' shape.
As mentioned above some people are not aware of 'L' as a drug slang, but it was actually used in one of rapper Nelly's song "Ride With Me" in which he said "If you wanna go and high with me - Smoke a "L" in the back of the Ben-z". Rapper Chief Keef also used it in his song "John Madden" in which he says "All my n*ggas grind Ls, Thats a smart thing"