"Didst thing" is not a commonly used phrase in modern English, but one could infer that it refers to an action or deed that has been done in the past. Antonyms, or words that have opposite meanings, for "didst thing" could include "undone action", "ignored task", "abstained from deed", or "left undone". These words imply a lack of action or effort, as opposed to something that has already been accomplished. Using antonyms for "didst thing" can help emphasize the importance of taking action and fulfilling one's responsibilities.