Showing off one’s proficiency with the axe before Lu Ban the Master Carpenter

Lu Ban was supposed to be a consummate carpenter in ancient times. It is said that he once carved a wooden phoenix that was so lifelike that it actually flew in the sky for three days. Thus it was considered the height of folly to show off one's skill with an axe in front of Lu Ban. 

古代有一个建筑和雕刻技术非常高超的人，名叫鲁班，木匠行里尊称他为祖师。传说他曾用木头制作了一只五彩斑斓的凤凰，能够在空中飞翔三天不掉下来。在鲁班门前摆弄斧子，当然显得有些自不量力了。

This idiom excoriates those who show off their slight accomplishments in front of experts.

“班门弄斧”这个成语，用来比喻在行家面前显示本领。