S-Letter Sayings  | 
It's believed that the origin of this phrase comes from the action of sticking a fork in meat, which is done regularly by people to check whether it has finished cooking.

After someone says this expression, it is common for the words "(someone or something) is done!" to follow. So, for example, "stick a fork in (him), (he's) done!" With this expression, the words in parenthesis are replaceable depending on who or what is being referred to.

* This new monitor is only a week old, but it looks like it is broken because it no longer turns on. Stick a fork in it.

* My cats have been outside for most of the day. I think they were very tired, because when I let them inside, they quickly fell asleep. Stick a fork in them, they are done.
S-Letter Expressions.
Phrases and Idioms

Note: Know Your Phrase has the meanings and origins for numerous expressions. If you have one that you're trying to find, you might want to use our alphabetical list to help you navigate around. There's also a list of idioms that are related to sports and animals, with a food one coming later, so if you're interested in that, give it a look. If you can't find a specific saying that you're searching for, perhaps come back another time since more will be added later. That's all for now!
KnowYourPhrase.com - An Alphabetical List of Common Expressions

The Meaning Of The Idiom 'Stick a Fork In It' And Its Origins

This is a fork. You can eat different foods with it. Yeah.
common idiom that's used to point out that someone or something is finished, or done.

Example: A man named Gilbert went to the gym. After 40 minutes of working out, he was exhausted and ready to go home. Thus, someone might say concerning Gilbert, "stick a fork in him, he's done."