Major League Soccer, the soccer league in North America.

MIB make it a point to always refer to it as "an MLS" instead of "the MLS." This is because of the MLS communications team, who over lunch asked Roger to not call it "the MLS" (10/20/2011 pod). From then on, MIB calls it "an MLS" because "they don't like the definite article" in MLS.

Davo tweeted about its origin here

Davo believe their new slogan should be "We're Working on the Middle!" (from the 6/21/2013 pod)