The M actually stands for Murderer! This is proven by the fact that the booklet for the first game actually mentions that the Toad's were turned into b
M is for Marinara, L stands for linguine, W is for "Wow pasta is good" and Γ stands for Gamma, showing Waluigi is actually a robot powered by a bird
According to Luigi, the M stands for "Moron" because he's a moron at golf and everything. The L stands for "Look at me, I'm so good at the game and everything"
According to Mario, the L stands for "Lunatic" because only an absolute lunatic would be worse than Luigi's driving skills. The M stands for "marvelous".