Sunk is the past participle of sink, used with helping verbs like has or had to show that something has gone down below the surface of water or another liquid. A rock tossed into a pond sinks immediately, and after it reaches the bottom, it has sunk. The Titanic sank after hitting an iceberg in 1912.

The word appears in an important phrase: sunk costs. These are resources like time, money, or effort you've already spent and can't get back. Imagine spending two hours building a tower of blocks that keeps falling over. Those two hours are a sunk cost. You can't recover that time whether you finish the tower or start something new.

People also say something has sunk in when they finally understand it. After your teacher explains a tricky math concept several times, it might suddenly sink in, and you get it. The idea has settled into your mind, like something heavy settling to the bottom of water.