An exclamation of surprise.

This originated in the North of England. It did travel to others parts of the UK during the 20th century, but not much further, and is little known in other parts of the English-speaking world. It is now less used than previously, although it is staple fare for any writer wishing to write a part for a stage Yorkshireman. Beyond that there's little more to tell. When it was coined and by whom, I don't know. Why the stairs and not the kitchen, the back-parlour or Belgium? I don't know that either.