Jam tomorrow or (older spelling) jam to-morrow is an expression for a never-fulfilled promise. It originates from Lewis Carroll’s 1871 book Through the Looking Glass and What Alice Found There. In the book The White Queen offers Alice “jam every other day” as an inducement to work for her:
"I’m sure I’ll take you with pleasure!" the Queen said. "Two pence a week, and jam every other day."
Alice couldn’t help laughing, as she said, “I don’t want you to hire ME – and I don’t care for jam.”
"It’s very good jam," said the Queen.
"Well, I don’t want any TO-DAY, at any rate."
"You couldn’t have it if you DID want it," the Queen said. "The rule is, jam to-morrow and jam yesterday – but never jam to-day."
"It MUST come sometimes to ‘jam to-day’," Alice objected.
"No, it can’t," said the Queen. "It’s jam every OTHER day: to-day isn’t any OTHER day, you know."
"I don’t understand you," said Alice. "It’s dreadfully confusing!"
I keep using this idiom on people who have no idea what I mean, which makes it kind of useless. I share with you now a simple way of summing up a complicated abstract idea! Go forth and propagate its usage!