During the Three Kingdoms period, Ma Su and his brother Ma Liang both served under Liu Bei. Ma Su enjoyed discussing military strategy and was highly regarded by Chancellor Zhuge Liang. However, Liu Bei felt his words were unreliable and said to Zhuge Liang before his death, "Ma Su exaggerates beyond reality and should not be given important responsibilities. Chancellor, you must be cautious."
Zhuge Liang nodded in agreement, but in his heart, he was not convinced.
After Liu Bei's death, Zhuge Liang devoted himself to assisting his successor Liu Shan in governing Shu Han. He allied with Eastern Wu to the east, resisted Cao Wei to the north, and when pacifying the southern tribal leader Meng Huo, he adopted Ma Su's strategy of "winning hearts is paramount, capturing cities is secondary," capturing and releasing him seven times to win over the people, permanently securing the rear.
In 227 AD, Zhuge Liang launched a campaign against the Wei kingdom. The following year, he captured three commanderies in quick succession. Alarmed by the crisis, Emperor Ming of Wei dispatched the general Zhang He with fifty thousand troops to resist, and ordered Sima Yi to join forces from the east to confront Zhuge Liang together.
Zhuge Liang was certain that Zhang He would come to seize the strategic stronghold of Jieting in Hanzhong, so he sent Ma Su as the vanguard to hold it. Before departing, Zhuge Liang repeatedly warned Ma Su not to be careless. Ma Su puffed out his chest and declared, "Rest assured, Chancellor. Since Jieting is the throat of Hanzhong, its terrain must be perilous. One man can hold it against ten thousand foes. Not just Zhang He—even if Sima Yi himself comes, I won't fear him!"
Still uneasy, Zhuge Liang appointed Wang Ping as a deputy general to assist Ma Su in defending the position. But upon reaching Jieting, Ma Su ignored Wang Ping's advice and foolishly set up camp on a wooded hillside. As a result, Sima Yi and Zhang He's forces surrounded them completely, slaughtering nearly all of Ma Su's twenty thousand troops. Only Wei Yan's timely rescue prevented Ma Su from being killed.
When Zhuge Liang received the map sent by Ma Su and Wang Ping from Mount Qi, he was horrified, but it was too late—the strategic outpost of Jieting had fallen, forcing Zhuge Liang to retreat to Yangping Pass. Ma Su, fully aware of his grave responsibility, wrote a letter from prison begging Zhuge Liang to care for his family, then took his own life.
Later, people used the idiom "to exaggerate" to refer to speech that is overstated and unreliable.
Source: *Records of the Three Kingdoms*, Book of Shu, "Biography of Ma Liang"
Meaning of the Idiom: Later, the Chinese idiom "言过其实" came to describe speech that is overstated and unreliable.