Duck soup is a colloquialism that means something is easy or simple. Some commonly used antonyms for the word "duck soup" include difficult, complex, complicated, challenging, and tough. These terms indicate that a task requires a considerable amount of effort, skill, or knowledge to complete successfully. Other antonyms for duck soup that might be used include arduous, daunting, demanding, formidable, and strenuous. Using such antonyms is useful when you want to convey the difficulty level involved in completing a task. Overall, antonyms for the word "duck soup" suggest a sense of challenge, difficulty, and complexity, rather than ease or simplicity.