Short for mustache. The term "mo" has gained notoriety and been propelled to international fame by the Movember movement.
Originated in Australia, Movember is an annual campaign to raise money for mens' health issues; notably prostate cancer and testicular cancer. Every year men across the globe begin November 1 with a clean shaven face and spend the month growing and grooming a mustache in an effort to raise awareness, and money.
Dude, check out that hideous Mo on school janitor.
Today when I took my Mo to the gas station a little girl screamed at the sight of us and hid behind her mother.