It is an outdated british slang phrase meaning "it is ready" or "it will be happen". It comes from old film industry slang, when the film was ready to be released.
Journalist: This movie took so long that it was almost forgotten...
Film director: Everybody made an excellent job, the film is in the can.
"She's in parties. It's in the can."
Bauhaus - lyrics She's in parties
The term or phrase that people say when they're in a large amount of trouble. The people that are in the can on a regular basis are probably the biggest scum bags you know. Also if your friends are always in the can, than there's a good chance you'll get dragged in every once in a while.
"Lucas got caught throwing a rock through someone's window, now hes in the can."
"Timmy hasn't answered his phone in a while. I think he's in the can."