The Online Idiom Dictionary explains it as "Don't make future plans on something that has not yet happened" and provides several similar idioms such as "Jumping the gun".
Sometimes it takes more than one astonishing event to convince someone of truth, and this was such a situation for Ahab, King of Israel. The Bible gives the following background:
King Ahab had not exactly been faithful to the God of Israel and as a result, his country had been in a severe drought. His wife, Jezebel had been a priestess of the Canaanite god Baal, who was supposed to bring rain. Elijah, the true God (Yahweh) prophet, had already wrought several miracles before God finally sent him to test Ahab.
Elijah called the Israelites to confront the priests and priestesses of Baal on Mount Carmel, and each group of worshippers built an altar and sacrificed animals ready for burnt offerings to God or to Baal. Baal's supporters prayed to Baal to light the fire, but no fire appeared. Then Elijah prayed to God, and fire appeared from above onto the altar of the Israelites.
The second test for Ahab is told in 1 Kings 20, when Benhadad, King of Syria, came up against Israel with an army many times that of Ahab's.
Benhadad sent emissaries to Ahab, purporting to try to avoid war, by simply asking him to stand down and hand over the Israeli nation to the Syrians. One requirement was that Benhadad's key people would take whatever took their fancy including women and children. Supported by his wise advisers, King Ahab was willing to acquiesce a little, but this was too much.
1 Kings 20 verse 10 is the classic "counting your chickens" statement, as Benhadad stated: "The gods do so unto me, and more also, if the dust of Samaria shall suffice for handfuls for all the people who follow me." (KJV) The New Living Translation reads: "May the gods bring tragedy on me, and even worse than that, if there remains enough dust from Samaria to provide more than a handful for each of my soldiers."
This usual explanation is that, in this passage, Benhadad is boasting that his army will overrun the Israelites so thoroughly that not even enough dust will be left to throw over his followers.
Ahab's reply to Benhadad statement was, according to the KJV: "Let him that girdeth on his harness boast himself as he that putteth it off:" New Living Translation has it as: "A warrior still dressing for battle should not boast like a warrior who has already won, and taken off his armour."
A prophet of the Lord came and assured Ahab that the battle was the Lord's (verse 12). Meanwhile Benhadad and his military cronies all got thoroughly drunk, "assuming" that the battle was already won, when in fact it had not yet been waged (verse 16). The rest is history "as they say" and Israel routed the Syrians.
Mark Tronson says that "counting your chickens before they hatch" is a lesson each and every new generation must learn for themselves. Journalists have rephrased it in reference to politicians, as "a week is a long time in politics." It is the same message. Nothing should be taken for granted as one never knows what is around the corner, whether that be in finance, business, politics, sport, entertainment, personal relationships, health or religion.
During the 1980s when M V Tronson was establishing chaplaincy appointments in professional sports around Australia, he quickly discovered that no two "sport organisations" were the same. Because one Sport organisation from the one Football code adopted chaplaincy ministry, this did not by any stretch of the imagination, mean the next one would do so, it was not a fait accompli. In other words, Mark Tronson realised that he could never count the chickens before they hatched.
The miracle of faith is often this: that God's hand is upon the events that set in train 'the process'. In a similar step of faith, he is finding the same process with his recently-acquired Laguna Quays Respite, which is available to Missionaries and Church personnel. Where the information and value of respite for missionaries and ministers has reached fertile ground, the word is spreading among the community of clerics and missionaries, and he has very quickly had numerous enquiries from people wishing to use the facility for respite.
"This is the alternative strategy to blindly 'counting the chickens'!" proclaims M V Tronson.
"I have found that if I place my faith in the Lord, and if I do as much planning as I can and locate people who can either make the decisions (as in the case of the sports ministries), or spread the word around the channels that the right community listens to (as in the case of Laguna Quays), then the planning I have done is sufficient for the Lord to set in train the 'incubation' process so that my chickens all hatch safely."
These are "only two of the many examples" of the numerous times during his various ministries when he has used careful planning as a bountiful springboard, then taken the necessary administrative or communication steps, and in faith, walked ahead seeking the wisdom of the Lord, yet exercising godly caution so as to never count the chickens before they hatched.