to feed a prisoner
昷 centers on providing sustenance to someone confined: feeding a prisoner. The idea is one of giving food or care to a person who cannot obtain it themselves.
昷 combines 日 (sun) over 皿 (dish/plate). The historical origin is uncertain, but it may relate to the idea of warmth or providing food under the sun, which connects to the meaning of feeding a prisoner.
Picture a dish (皿) placed under the sun (日) with food on it, being given to a prisoner. The sun warms the meal as it is handed over: feeding a prisoner.
For オン, imagine turning on a warm sun-lamp over a dish of food for the prisoner: on -> オン, and the warmth helps feed them.