What you do when you're talking with someone about something that both of you know a great deal about. Two engineers can talk shop about their projects, two musicians can talk shop about any piece of music you throw at them, two Everythingians can ramble on and on about Everything, you get the idea. Shop talk is often linguisticly characterized by code, or a vocabulary which is unique to the subject. It sounds "greek" to everyone else. The most codified shop talk is drumspeak, which has few actual English words, but can still convey ideas, especially musical ideas.