Mare-nest is a relatively uncommon word that refers to a dream or fantasy that is unlikely to come true. It is often used to describe a fanciful idea or plan that has little chance of success. There are several synonyms for mare-nest, including pipe dream, reverie, daydream, wild idea, and flight of fancy. Each of these words conveys the same sense of a whimsical thought or imagined scenario that is not grounded in reality. While mare-nests can be fun to indulge in from time to time, it's important not to get too caught up in them and to stay focused on achievable goals.