Of, or pertaining to, being very high from reefer.
The "Mo" means extremely/very.
Origin: Was created by several students who attended Puyallup High School ('05-'06), to make fun of another student. Needless to say, it has spread and become vastly popular.
"Man this pot is mo-dank."
"That movie was mo-gay."
"Dude you're trippin' mo-balls."