【Explanation】: This idiom metaphorically describes showing off one's abilities in front of an expert. It is often used as a modest expression.
["A mound of earth by the Caishi riverside, Li Bai's name has towered for a thousand years;
Countless poems left by passersby, wielding a giant axe before Lu Ban's door."]
This poem was written by Mei Zhihuan of the Ming Dynasty to commemorate Li Bai's tomb. Li Bai (701–762), styled Taibai, was a renowned Tang Dynasty poet, famous for his love of wine and known as "Li the Banished Immortal." There are numerous mythical legends surrounding his death. One account claims that in his later years, Li Bai traveled to places such as Dongting, Yueyang, and Jinling (modern-day Nanjing). One day, his boat anchored by the Caishi River (Caishi, now in Dangtu County, Anhui Province, bordering the Yangtze River). That night, the moon shone brightly as day. Li Bai drank freely in his boat, becoming thoroughly intoxicated. Seeing the moon's reflection in the water, he leaned over to catch it and fell into the river, drowning. Another even more fantastical tale says that suddenly, great waves arose in the river, a giant whale leapt from the water, and two celestial boys appeared, holding banners, inviting Li Bai to ride on the whale's back, with heavenly music leading the way as they ascended into the sky. While these stories are clearly mythical and not to be believed, they have led to the creation of many scenic spots in Caishi. Besides Li Bai's tomb, there are also the "Banished Immortal Pavilion" and the "Moon-Grabbing Pavilion," attracting countless tourists. Some visitors, despite having poor literary skills, attempt to feign refinement by composing and scribbling crude verses on Li Bai's tomb, which is quite laughable. Mei Zhihuan's poem specifically mocks such tourists. He believed that scrawling bad poetry on the tomb of a great poet was exactly like "wielding a giant axe before Lu Ban's door"—a clear case of overestimating one's own ability.
Lu Ban (also known as Gongshu Ban or Lu Ban), was said to be a native of the State of Lu during the Warring States period. He was a master craftsman renowned for his skill in making intricate tools and was known as the "ingenious man." Folk tradition has long revered him as the founding ancestor of carpenters. Who would dare show off their axe skills in front of Lu Ban? Thus, attempting to display one's abilities in front of a true master—a presumptuous and ridiculous act—is called "wielding an axe before Lu Ban's door," shortened to "Banmen Nongfu" (showing off before an expert). It carries a similar meaning to the common saying, "waving a broadsword before Guan Gong" (Guan Yu, the martial deity).
The Tang Dynasty writer Liu Zongyuan, in the preface to his work "Poems in Harmony by the Wang Brothers," wrote: "Wielding an axe at the doors of Ban and Ying—that is sheer impudence." (Ying refers to another ancient master of the axe; see "Fu Zheng" for more details. "Qiangyan" means to have the audacity or thick skin.) Thus, the idiom "Banmen Nongfu" not only originated from Mei Zhihuan's poem in the Ming Dynasty but already had its prototype as far back as the Tang Dynasty.
This idiom is sometimes used modestly to express one's reluctance to display skills in front of an expert.