Used to describe pyschadelic music with progressive tone changes, ie, progessive music. This music is so progressice that it is foggy and the notes kind of just float mid-air, barely changing at all. Also can be used when touting progressive music that you feel is exceptionally trippy. A stoner type term.
"that album is really proggy when you hear it on headphones"
Packed with progressive elements, esp. regarding the genre adjective which applies to music - i.e. progressive rock, progressive metal, progressive folk, etc. "Progressive" generally connotes virtuosic instrumentation, incorporation of exotic sounds/instruments, use of uncommon time signatures, unusual/abstract lyrical subject matter, etc.
I'm sort of OK with Coheed and Cambria, I guess, but they are not nearly as proggy as Dream Theater or The Mars Volta. They sound like a less-proggy version of Rush, except Rush didn't have an emo influence.
Proggy means Program.
Open up that proggy and do stuff with it.