[詞典] teach one's grandmother (how) to suck eggs; display one's slight skill before an expert; teach fish to swim; show off in the presence of an expert;
[例句]先生大名，如雷灌耳，小弟獻醜，真是班門弄斧了。
Your great name long since reached my ears like thunder. I am ashamed to show off my incompetence in the presence of an expert like yourself.