The phrase "knocked oneself out" generally means to make a lot of effort, work hard or be exhausted. Antonyms for this phrase would involve terms that have the opposite meanings, such as being calm, relaxed, or energetic. Terms like lazing around, slacking off, or being lethargic would be the opposite of "knocking oneself out." Additionally, the antonym may vary depending on the context and the usage of the phrase. Still, generally, any word that refers to the opposite of exerting effort or being drained would serve as antonyms for "knocked oneself out.