The phrase "be on feet" implies being physically upright and mobile, and therefore, has various antonyms. The opposite of being on feet is lying down, sitting, or being in a reclined position. These actions indicate the absence of physical activity, and of being stationary. Being sedentary is an antonym of being on feet. Other antonyms of this phrase include being incapacitated or being unable to move for some reason. It is also the opposite of being energetic and active. In conclusion, be on feet has various antonyms, depending on the context and circumstances, but they all relate to the absence of physical activity and being stationary.