The definition of couldn't is could not.
An example of couldn't is a woman saying she was not able to wear white to a wedding.
- "I couldn't sleep," she said.
- I was so excited that I couldn't sleep, so I got up and dressed.
- You couldn't see a thing.
- "Yes. I couldn't find a better friend than Alex, could I?" she said.
- If he has custody, she couldn't get the money.