How did 东西 come to mean "something" in the expression "吃东西"?
This is a good question about etymology of Chinese word. Most native speakers are not interested in etymology as they can speak out of habit while non-native speakers are more curious on how the sense of words evolves. Back to your question, there are nice articles on both Wikipedia and Baidu Baike that explains the etymology of "东西". Since there are acutally quite a few explanations for this one, so I cannot translate them all here. But I can help you if you have trouble to understand particular sentences.