Dinna fash yersel
It means do not let it bother you, it is just not worth it. Of course, Freepers like myself, and there are many, exude politeness, knowledge and reasonable discourse.
Ducking quickly and closing out and taking the family dog out to the park. (laughs).
And the Olde English origin was "Do not fash thyself unduly", and yes it means pretty much what your version does. Not sure which one came first or if it is merely a mixing of words but it is not a difficult concept to grasp.