The term "L" originally referred specifically to an L-Plate joint. For those who do not know, an L-Plate joint is made by sticking two rolling papers perpendicular to each other in the shape of an "L", allowing for a larger joint.
Nowadays, the term "L" refers to both a blunt (marijuana cigar) as well as L-Plate joints.
The evolution of the term either came from blunts rolled with El Producto cigars, or due to the size of an L-Plate joint being larger than the average joint and possibly similar to a blunt.
Dude: "Yo son come smoke an L with me."
Dude #2: "Aight."
(Also, when you hear rap songs where people are smoking "Ls", they are almost certainly referring to blunts rather than L-Plate joints, unless they specifically mention that they are smoking joints or spliffs at the same time.)