Antonyms are words that have opposite meanings to a given word. The word "zestiest" can be described as full of enthusiasm, energy, and excitement. The antonyms of "zestiest" include boring, dull, lifeless, unenthusiastic, uninspiring, and lethargic. These words have meanings that are contrary to those associated with "zestiest." While "zestiest" is a word that connotes vibrancy and energy, the antonyms describe a lack of enthusiasm, inspiration, and life. Therefore, if you are looking for a word that is the opposite of "zestiest," you can use any of the above antonyms to convey a contrasting meaning.