Please don't read the tittle and read the Phrase 2 times with natural speed before read the situation about it. ^^ Thank you very much.
"No, I couldn't possibly. I'm so stuffed."
Situation :
You just ate a big meal at a friend's house. Your friend offers you some more food, but you're too full. You say this to refuse.
That phrase is old and sounds too uptight now. More likely is:
"No thanks, I'm full"
Or where I am from :
"Nah, got no room"