班门弄斧 — “To show off one's slight skill before an expert”

Lu Ban was a craftsman from the State of Lu during the Warring States period. He was highly skilled in making sophisticated tools and was known as a 'skillful person.' People in the folk tradition have always revered him as the ancestor of carpenters.

'Wielding an axe before Lu Ban's door' means showing off one's axe-wielding skills in front of Lu Ban, the grand master. It implies displaying one's meager abilities before an expert, a presumptuous and laughable act. This is similar to the saying 'showing off one's sword before Guan Yu,' meaning to display one's martial arts skills in front of a legendary warrior.

In fact, the idiom 'Ban Men Nong Fu' had its rudimentary form as early as the Tang Dynasty. The literary scholar Liu Zongyuan once wrote in a preface: '操斧于班郢之门，斯颜耳！' (To wield an axe before the gates of Ban and Ying, how thick-skinned!) This means that showing off one's axe skills in front of Lu Ban and Ying Ren (another master of axe skills) would be incredibly shameless. This idiom is sometimes used as a self-deprecating remark, indicating one's reluctance to show off a small skill in front of an expert.