The phrase "stick a fork in it" is often used as an idiom meaning that something is a lost cause and to give up on something. It is also sometimes used when someone has finished a particularly difficult task to indicate exhaustion.
This idiom is often used in the context of sports. "Stick a fork in it" likely comes from the practice of sticking a fork into meat to see if it has finished cooking and is ready to eat. "Stick a fork in it" is also sometimes used as an exclamation when somebody does not want to discuss something or to prevent the other person from talking about certain topics.