The phrase "out on feet" typically means that someone is exhausted or tired. Here are some antonyms, or words with the opposite meaning, to describe someone who is not tired: energetic, invigorated, fresh, lively, wide-awake, alert, perky, rejuvenated, strong, and robust. These words can be used to describe someone who has had enough rest or has been re-energized by some activity or event. Antonyms can help to better understand the meaning of a word by showcasing its opposite. In this case, using antonyms for "out on feet" can give us a better understanding of what it means to be well-rested and energized.