When you empty something out with a liquid, you flush it. You might also be flush with cash, meaning you have a lot of it. Just don't flush that cash down the toilet!
Your face can flush, and the sky can flush at sunset when it glows with shades of pink. You can also use flush as an adjective to mean "lined up" or "level," like when you hang a medicine cabinet in your bathroom, making sure it's flush against the wall. Yet another meaning of a flush is a peak or boom — a professional poker player might describe her period of winning game after game as a flush.