A grade of marijuana higher than a normal commercial, while still is a type of commercial weed. However, is a step above normal weed; a step below kind bud. Hence the name "middies", as it is in the middle of normal and very good.
This type of bud is normally very dank and not compressed, such as normal commercial.
A middy is shot-form for a "midterm". Used by students at the University of California, Berkeley - especially since they have so many midterms. The term "middy" was inspired by Lil Wayne's hit song, a "Milli", but given educational and practical value.