Jam tomorrow or jam to-morrow is an expression for a never-fulfilled promise. It originates from Lewis Carroll's

Lewis Carroll

Charles Lutwidge Dodgson , better known by the pseudonym Lewis Carroll , was an English author, mathematician, logician, Anglican deacon and photographer. His most famous writings are Alice's Adventures in Wonderland and its sequel Through the Looking-Glass, as well as the poems "The Hunting of the...

 1871 book Through the Looking Glass and What Alice Found There

Through the Looking-Glass

Through the Looking-Glass, and What Alice Found There is a work of literature by Lewis Carroll . It is the sequel to Alice's Adventures in Wonderland...

. In the book The White Queen

White Queen (Through the Looking Glass)

The White Queen is a fictional character who appears in Lewis Carroll's fantasy novella Through the Looking-Glass.-Plot:Along with her husband the White King, she is one of the first characters to be seen in the story. She first appears in the drawing room just beyond the titular looking-glass as...

 offers Alice "jam every other day" as an inducement to work for her:
The Queen's rule is a pun on a mnemonic

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 for remembering the distinction between the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 words "nunc" and "iam" (sometimes written "jam"). Both mean "now", but "nunc" is only used in the present tense, while "iam" is used in the past and future tenses, much like one use of the English word 'then'. (It is not clear if Carroll invented this mnemonic or was merely quoting it.)

Usage

In more recent times, the phrase has been used to describe a variety of unfulfilled political promises on issues such as tax

Tax

To tax is to impose a financial charge or other levy upon a taxpayer by a state or the functional equivalent of a state such that failure to pay is punishable by law. Taxes are also imposed by many subnational entities...

, and was used by C. S. Lewis

C. S. Lewis

Clive Staples Lewis , commonly referred to as C. S. Lewis and known to his friends and family as "Jack", was a novelist, academic, medievalist, literary critic, essayist, lay theologian and Christian apologist from Belfast, Ireland...

 in satirising the extrapolation of evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 from biological theory to philosophical guiding principle, in his 1957 Hymn to Evolution, a poem based on Lead us, heavenly Father, lead us and set to the same tune, Mannheim

Mannheim

Mannheim is a city in southwestern Germany. With about 315,000 inhabitants, Mannheim is the second-largest city in the Bundesland of Baden-Württemberg, following the capital city of Stuttgart....

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