Bay slang... it means how many people can ride on those short yellow buses.. also means goin dumb, gettin hyphy, stupid, and retarded... Popularized by The Federation's single "18 Dummy", hence the name.
Related to Bay Area Lingo such as: getting Hyphy, going dumb,and getting retarded; and also serves the same definition as being under influence of any mind altering drugs, or alcohol.
It's main emphasis is on 1800 Jose Cuervo Tecquila
Sometimes other numbers are used as Going 24 Dummie, 32, and so on.