Meaning: a promise which is never likely to be kept.
Origin: Jam tomorrow derives from Lewis Carroll's Though the Looking Glass and What Alice Found There (1871), in which the White Queen offers Alice "jam to-morrow":
- 'I am sure I'll take you with pleasure!' the Queen said. 'Twopence 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 cannot,' said the Queen. 'It's jam every OTHER day: to-day is not any OTHER day, you know.'
'I don't understand you,' said Alice. 'It's dreadfully confusing!'
The popularity of the Alice books lead to the wider use of jam and other phrases were coined in the early 20th century. Anything cushy or rewarding might have been described as 'with jam on it'. For example, this item from Fraser and Gibbons' Soldier and Sailor words and phrases, 1925:
- "‘You want jam on it?’, i.e.: ... you expect too much."