According to the website in their comment it apparently stands for "Tension Myositis Syndrome (TMS)" (https://www.tmswiki.org/ppd/An_Introduction_to_Tension_Myosi...)

It stands for tension myositis syndrome. This is a good introduction to it (1)

(1) https://www.tmswiki.org/ppd/An_Introduction_to_Tension_Myosi...

as others have said, tension myositis syndrome. The theory is supressing uncomfortable thoughts results in the body instead trying to communicate them as pain.

I personally think its quack science, but it seems to work for some people, so that is what matters.