During the Spring and Autumn period, Duke Xian of Jin wanted to conquer the state of Guo, but his army had to pass through the state of Yu. Worried that Yu would refuse passage, the Duke pondered endlessly but found no solution.
Grand Physician Xun Xi learned of this and advised Duke Xian of Jin, "Your Majesty, in my humble opinion, we should offer fine horses and jade as tribute to bribe the Duke of Yu, thereby borrowing a path through Yu to attack and annex Guo."
Duke Xian of Jin remained silent. The fine jade and the swift steed were his two greatest treasures, and he was truly reluctant to part with them.
"Your Majesty, a fine horse and a beautiful jade can be reclaimed, but the state of Guo cannot be easily won." Xun Xi, sensing Duke Xian of Jin's hesitation, offered this counsel.
Duke Xian of Jin pondered for half a day before finally accepting his advisor's counsel.
The jade was unparalleled in beauty, and the steed was magnificently unmatched—King Yu of Yu could not bear to put them down, clicking his tongue in admiration. He readily agreed to clear a path for the Jin state to attack the Guo state.
The State of Jin effortlessly conquered the State of Guo, swallowing it whole in a single campaign.
Duke Xian of Jin remained gloomy, showing no joy in victory. Xun Xi guessed his thoughts and said, "Your Majesty, I once said, 'The fine horse and beautiful jade can be taken back.' Now that the state of Yu is defenseless, why not strike and swallow it whole, reclaiming both treasures at once?"
Duke Xian of Jin was overjoyed and immediately ordered an attack on the state of Yu.
On the journey back, the state of Jin indeed went on to conquer the state of Yu.
Minister Xun Xi led the fine horse forward, holding a beautiful jade in his hands, and said to Duke Xian of Jin, "Your Majesty, the jade is still the same jade, but the horse's teeth have grown."
Later, people often used "horse teeth long" to refer to the increase in age, and "horse teeth grow in vain" to describe that one's age has increased but their knowledge or achievements have not, wasting time, as a sign of modesty.
Source: *Guliang Zhuan*, Chapter "Duke Xi, Second Year"
Meaning of the Idiom: Later, the Chinese idiom "马齿徒增" came to describe horse teeth long refers to the increase in age, and horse teeth grow in vain describes that one's age has increased but their knowledge or achievements have not, wasting time, as a sign of modesty.