The word "out-does" means to surpass, exceed or outperform someone or something. Its antonyms are words that carry the opposite meaning, such as underperforms, lags behind or falls short. To underperform is the opposite of outdo because it means to perform less than expected or required. Lagging behind and falling short both mean failing to meet set standards, expectations or goals. Thus, if someone or something is said to "out-do" another, then the other is said to "underperform," "lag behind" or "fall short" of their standard. Knowing the antonyms of "out-do" broadens one's vocabulary and helps in articulating the exact expression required in a conversation or writing.