It's a humorous and emphatic way to express a strong refusal or disagreement. The phrase is often used in a lighthearted or playful way, though it can be used seriously as well.
Here are some examples of how it might be used:
* "Are you going to jump off that cliff?" "Not by the hair of my chinny chin!"
* "Do you think you can finish that whole pizza?" "Not by the hair of my chinny chin!"
* "Would you ever steal from your best friend?" "Not by the hair of my chinny chin!"
The phrase itself isn't very old, and it's unclear exactly how it came about. It's likely a combination of two common phrases: "not by a hair's breadth" and "chinny chin chin".
It's a fun and memorable phrase that is sure to get a chuckle from those who hear it!