Answer:
Pain is weakness leaving the body is a quite unhealthy inspirational quote used to pressure people to work harder than they should.
While pain is the body’s national response to overload, and a mental signal for you to stop, people with ulterior motives will argue that pain, in fact, is a good thing.
Even if we could use some type of high tech camera to see the physical shape of “weakness”, most scientists agree that “weakness” does not leave the body when you feel pain.
One part that can be considered true, in some cases, is that some levels of pain can make you stronger, making you less prone to weakness in the future.
This does not mean that you should pressure yourself way across your pain limits in order to not be weak, as it will likely make things worse.
As a society we are raised with the idea that to quit equals weakness. No matter how much pain you're in, no matter how bad things are, the one thing you NEVER do is GIVE UP.