Dream on is a phrasal verb which means you have to think of it as if it were a totally new piece of vocabulary. In conversation : Jenny : " May I have the last slice of pizza?" You : " Dream on!" It would be the same as saying "no way". Sometimes people say " keep dreaming" which means a similar thing. You can remember like this.... If you want something it's like you are "dreaming" about getting it and someone says... "No way or keep dreaming... Or dream on" as a way of saying no, it's not going to happen. I hope that answers your question.[emoji]  If you want to improve your English even more why not take a look at my teaching profile and classes. I'm offering $5 trial lessons right now! [emoji] My classes are fun and relaxed, I know you will enjoy them.

dream on = you're dreaming. This means "whatever you're thinking about won't happen." You think the banks will loan you the money to buy that big house?? Dream on!