満腹（まんぷく） means "stuffed" and you should say it right after a large meal, fall out of your chair, and pass out on the spot.
I learned this word during my first year of studying Japanese at University because I had a tendancy (like all poor college kids) to gorge myself at events with free food. A Japanese exchange student whom I had befriended was kind enough to laugh at my drooping face and exclaim "満腹！" very loudly which caused a number of people to look over in our general direction though they had no idea what he was saying.
It's also polite to say this word because it conveys a sense of "damn that food was good so I ate too much" with it. I recommend using this whenever you're sat down at a homestay and the dad or mom makes you a large meal. They will chuckle because this is kind of a slang term (yet not dirty in any way). Just one more way to make those in-roads towards lost of Japanese friends and cross-cultural communications!