班门弄斧 (bān mén nòng fǔ)
In ancient times, Lu Ban was supposed to be a consummate master in construction and sculpture. Carpenters respect him as ancestor master. It is said that he once carved a colorful 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.This idiom excoriates those who show off their slight accomplishments in front of experts.