What does yam mean in the Yanqing dialect of Beijing?

We are from Yanqing and let me tell you. The locals refer to potatoes as yams. This is an unwritten rule left over for many years. What you mean by his true expression is: You are a yam. The original meaning is: You don't quite understand. To put it bluntly, he is just a bastard. It's demeaning, but it's also a joke. Hope this helps.