Don’t count your chickens before they hatch is an old adage which means, broadly, don’t act as though you have something before you actually have it. It can be used to refer to any number of things, from physical objects to events that have not yet come to pass.
An example might best illustrate when the phrase would be used. Let’s say a person applies for a job, with a high-paying salary, and finds out they have been hired. They immediately go out and buy a new car, even though they don’t currently have the money to afford the monthly payments. They assume that with their new salary they’ll be able to afford it. At this point, it might be appropriate to tell them, “Don’t to count your chickens before they hatch.” This is because they don’t actually have the money; they are relying on something that hasn’t yet come to pass.
The adage comes from an old fable, usually attributed to Aesop. Aesop was a Greek slave who wrote an enormous corpus of fables, each meant to illustrate a single point, and often ending with a one-line adage such as Don’t count your chickens before they hatch. He lived in the 6th century BCE, and hundreds of fables have been attributed to him, although it is uncertain how many were actually written by him.
The fable that the moral comes from is a simple one, usually only a few lines long. The general story goes as follows:
A young milkmaid was walking to the village, carrying a pail of milk on her head to sell at market. While she was walking she started thinking about what she would do with the money she made from selling the milk. “I will buy some chickens from Theonia,” she thought, “And when they lay their eggs each day I shall sell them to Liates. With that money I will be able to buy the finest clothes, and when I go to the market the boys will all look at me with affection. Cassandra shall be jealous, but I shan’t care, and when she looks at me I will toss my head like this.” And she tossed her hair back, spilling the milk all over the ground. She returned home and told her mother what had happened, and her mother responded, “Ah, my child. Do not count your chickens before they hatch.”
The fable first appeared in English in the late-16th century, in the form: “My chickings are not hatcht I nil to counte of him as yet,” and not long after as something a bit closer to our own, as “I would not have him counte his chickens so soone before they be hatcht.”