The phrase "in your dreams" is a dismissive way of telling someone that what they are suggesting is unrealistic or impossible. However, there are several other synonyms that can be used to convey the same meaning. For instance, one can use the phrase "not a chance" or "no way." "You must be kidding me," "get real" or "that's a joke" can also be used. Other common phrases include "dream on," "wishful thinking," or "fat chance." Generally, these phrases are used to express skepticism about a suggestion or a proposal. Overall, there are many ways to convey the same message as "in your dreams".