The phrase "a shot in the arm" typically refers to something that revitalizes or encourages someone, giving them a boost of energy or enthusiasm. Some possible antonyms for this phrase might include "a letdown," "a disappointment," "a setback," "a discouragement," or "a source of disillusionment." Rather than inspiring or motivating someone, these antonyms could have the opposite effect, leaving someone feeling deflated, demoralized, or disheartened. Whether it's a failed project, a missed opportunity, or a negative critique, these sorts of setbacks can be difficult to overcome, but with time, perseverance, and a positive attitude, individuals can rise above them and find new sources of inspiration and encouragement.