During the Spring and Autumn period, the state of Lu had a high official named Meng Wubo. He was notorious for speaking irresponsibly, often making reckless promises, and using his sharp tongue to mock others with biting sarcasm. His colleagues would glare at him in disdain but felt powerless to stop him, and even the ruler of Lu grew increasingly displeased with his behavior.
During a banquet at the palace, Duke Ai of Lu hosted his ministers, and Meng Wubo attended as usual.
At that moment, another official named Guo Zhong happened to be seated beside Meng Wubo. Guo Zhong was quite overweight and favored by Duke Ai of Lu, which often aroused Meng Wubo's jealousy and mockery.
At this banquet, Meng Wubo, using the opportunity to toast, sarcastically remarked to Guo Zhong, "Minister Guo, what have you been eating to grow so fat?" Just then, Duke Ai of Lu, seated on Guo Zhong's other side, heard Meng Wubo's words and, displeased, interjected, "He has eaten his words too often—how could he not be fat?"
On the surface, this remark meant: "He has swallowed all your words—how could he not grow fat?" But its true implication was: "A man who breaks his promises—how could he not grow fat?" Since Meng Wubo was notorious for going back on his word, he instantly realized that Duke Ai of Lu was using this jibe to mock his habit of "eating his words." At such a grand banquet, being ridiculed by the ruler himself, Meng Wubo's face flushed crimson with shame.
The other ministers at the banquet, catching the hint, burst into knowing laughter.
Later, the idiom "to grow fat on broken promises" came to describe someone who breaks their word and only seeks personal gain, while "never break a promise" means to steadfastly keep one's word.
Source: *Zuo Zhuan*, "Duke Ai, Year 25"
Meaning of the Idiom: Later, the Chinese idiom "决不食言" came to describe how someone who breaks their word and only seeks personal gain, while "never break a promise" means to steadfastly keep one's word.