"Giddap", a unique and mildly archaic interjection, is used as a command to urge a horse or any livestock to move forward. However, if you're looking for some alternative expressions to add a touch of variety to your vocabulary, several synonyms can effectively replace this term. For instance, you may consider using "gee up" or "hup" as interchangeable terms. Similarly, you can employ phrases like "go on", "move along" or "get going" to convey the same meaning. These substitutions can bring a refreshing dynamism to your language while maintaining the original intent of encouragement or prompting an animal to start moving.