Fastnet is a proper noun that refers to a famous lighthouse situated off the coast of Ireland. However, the term can also be used as a synonym for quick, swift, or rapid. Some possible synonyms for Fastnet include promptly, expeditiously, briskly, speedily, and fleetly. Fastnet can also be replaced with the words hastily, abruptly, and suddenly. It is important to note that synonyms are not always interchangeable and may carry different connotations. Therefore, it is essential to consider the context and intended meaning of the word before using a synonym. Nevertheless, expanding one's vocabulary by exploring synonyms can enhance communication and add variety to one's writing.