māʻona - full as in satisfied one's hunger
piha - full, as opposed to empty - like a container.
Wow. So is piha a slang term, as in piha ka 'opū? My pidgin world is crumbling.
Not really slang. Piha literally means full. Saying "Piha ka 'opū" you are saying the stomach is full (of food). You cannot use piha for the prompt because the word full in that context means their hunger is satisfied, which is māʻona not piha.