The phrase "most fortifying" refers to something that strengthens or invigorates the body or mind. Antonyms for this phrase would, therefore, refer to things that weaken or drain energy. Words that could be used as antonyms for "most fortifying" might include weakening, debilitating, exhausting, or draining. These opposites describe actions or environments that drain your energy, making you feel tired or weakened. For example, staying up late, eating junk food, or being exposed to constant stress can be considered antonyms of "most fortifying" activities like getting enough sleep, eating healthy foods, and engaging in exercise or other stress-relieving activities.