1. To sink below the surface of the water: The ship struck a reef and foundered.
2. To cave in; sink: The platform swayed and then foundered.
3. To fail utterly; collapse: a marriage that soon foundered.
4. To stumble, especially to stumble and go lame. Used of horses.
5. To become ill from overeating. Used of livestock.
6. To be afflicted with laminitis. Used of horses.

Usage Note: The verbs founder and flounder are often confused. Founder comes from a Latin word meaning "bottom" (as in foundation) and originally referred to knocking enemies down; it is now also used to mean "to fail utterly, collapse." Flounder means "to move clumsily, thrash about," and hence "to proceed in confusion." If John is foundering in Chemistry 1, he had better drop the course; if he is floundering, he may yet pull through.