Heppest is a word that means cool or fashionable. Synonyms for the word are groovy, trendy, stylish, hip, slick, and with-it. In modern times, being hep or heppest means being in the know and understanding the latest trends and technologies. Some other synonyms for heppest include happening, up-to-date, current, and in-style. As language and culture continue to evolve, new words will continue to emerge that capture the essence of being hep. However, for now, these synonyms effectively describe what it means to be heppest and stay on top of what's popular and sought after in society.