I don't know what the middle initial stood for in John M.Warbington's name. Also, I've not yet found any indication of his brother, Jacob Warbington, having a middle name, although it is interesting to note that he had the same given name as his uncle, Jacob B. Warbington, Sr. I probably never will know, as he died much younger than his brother, John.
With the way folks back then often used their mother's maiden name as their middle name, that might have been the case. But, since I don't know what Abigail's maiden name was, I can only speculate that might have been what they did!