Here is the exact definition:
To express extreme exasperation, shock, or dismay. A magnification of fuck me. Metaphoric exaggeration of being fucked while trying to run.
No clue- my guess would be they think you've lost your mind and don't want to be anywhere around nor associated with you in the future
It's almost like, Holy ****, or Damn it...I say **** me runnin' backwards. With the unbelievable crap I see and put up with, it's likely you'll hear that from me... a lot.
“Sometimes the questions are complicated and the answers are simple.”
― Dr. Seuss