The word "ex-celling" indicates the act of surpassing or excelling in some aspect. However, there are several antonyms for the word, which signify the opposite of excelling. Some of the prominent antonyms include underperforming, failing, floundering, struggling, and lagging. These words indicate scenarios where the performance or the output is subpar, and the individual is not meeting the expected standards. For instance, a person who is underperforming is failing to exceed expectations or deliver exceptional results. Similarly, lagging indicates that an individual is falling behind and struggling indicates difficulty in matching the performance of others. In summary, antonyms for excelling offer a diverse range of words that depict a spectrum of negative performance outcomes.