The word "unhastiest" refers to the absence of haste or the quality of being slow. Its antonyms are words with opposite meanings, such as "swift," "speedy," "hasty," "quick," "fast," "prompt," and "expeditious." These words suggest a sense of urgency or swiftness in performing a task or completing an action. "Unhastiest" can also be antonymized by words that convey laziness or sluggishness, such as "leisurely," "idle," "inactive," "sluggish," "listless," and "slow-moving." Therefore, depending on the context, choosing an appropriate antonym for "unhastiest" can help convey a specific tone or mood in writing or speech.