Displaying one's slight skill in front of a master
班门弄斧
bānménnòngfǔ

It literally means "show off one's skill with the axe before Lu Ban (鲁班), the master carpenter."

The Story:
In the ancient times, there was a man named Lu Ban who was renowned for being a master carpenter. Many people have told the story about the time he once carved a wooden phoenix so lifelike that it actually flew the sky for three whole days. It was considered to be exceptionally rude and ridiculous to show off one's skill with an axe in front of Lu Ban.