The phrase "to eat someone's lunch" means to take away their advantage, to defeat or surpass them in a competition. It is an idiomatic expression that refers to being more successful than another person or group, often by taking something valuable from them. This can happen when a new competitor enters the market and offers better products or services, causing established businesses to lose customers or sales. In other words, it means to outperform someone in such a way that they suffer loss or setbacks.