MIDI is an acronym for Musical Instrument Digital Interface. It is basically a sequence programming protocol, commonly used for composing music. The concept for MIDI was invented in the 80's when there was no common communication standard between synthesizers. Over the years, it has gotten a reputation for sounding like crap. What people keep forgetting, is that MIDI isn't responsible for producing sound. The synthesizer is. Most music teaching programs, like Synthesia, use MIDI files.
The most common MIDI files utilize General MIDI 1.0, although there exists MIDI 2.0, with HD-MIDI being in development right now. Unfortunately, GM 2.0 isn't supported by a lot of synthesizers.

to sport a boner, fly the sail at full mass, rock hard, etc.

Musical Instrument Digital Interface

Some okay weed, it kind of sucks.

Kick ass music, often used in DOS Games to show how kick ass they are

Musical Instrument Digital Interface
Gangster-ass ways of letting music devices communicate between one and another.
Except it has serious bottlenecks in speed - which fucks up its entire purpose.

When something sounds hollow, without skill, and computerized with no emotion. Usually sounds very muffled with a high level of MID's.