The phrase "my foot" is an informal expression used to express disbelief, disagreement, or rejection of a statement or claim. It is often used to indicate that one strongly disagrees with or finds something to be untrue or absurd. Here's an example of how it can be used in a sentence:

Person A: "I can run a marathon in under two hours."
Person B: "Oh, please! My foot! That's highly unlikely."

In this example, person B is expressing skepticism or disbelief in response to person A's claim. The phrase "my foot" is used figuratively to dismiss the statement as false or exaggerated.