Is Yamna or Yamnah an Arabic name what is its meaning?

"Yamna" and "Yamnah" are not Arabic names. They are likely variations of the name "Yamna" or "Yamnah" which is of Hebrew origin.

Yamna or Yamnah means "Right" or "Right Hand" in Hebrew. It is a feminine name and is sometimes considered to be a name that refers to the Right Hand of God.

It's possible you may have heard the name in a context where it was being used by Arabic speakers, but it's not a common Arabic name.