The phrase "didst best" is considered archaic English and means "did one's best." Some antonyms that could be used instead include "underperformed," "fell short," or "failed to excel." Other ways to express the opposite of "didst best" could be "halfhearted effort," "inadequate attempt," or "uninspired performance." However, it is essential to note that "didst best" implies someone attempted to do their best, while antonyms could suggest a lack of effort or skill. Therefore, context is crucial when choosing antonyms for the phrase "didst best.