^It's usually used when people are passing around a bowl, and whoever takes the last hit then says that the bowl is cashed/empty/done/etc. In America we have slang for everything.
An empty bowl is not cashed. Like I wouldn't refer to a pipe before I smoked it as being cashed, but if I had just taken the last hit off a bowl and the bowl is empty I would say that the bowl is cashed.
Or if I'm in a circle and someone is sitting around holding the bowl without hitting it for a while I might ask "is that cashed?" to see if it's gone or if they just zoned out. The term has its uses.