Pirates have many nations of origin and cover a vast period of time including the 21st century. I think you may find during your research many conflicting stories. I suggest that more then one source for the term "Jolly Roger" is likely to be the truth since languages get rearranged as time passes. I know of an island named "Bois Blanc" which is french for White Wood. The current residents of Bois Blanc pronounce it "Boblo" You may find that a strange way of pronouncing it but they think nothing of it at all. It has been that way so long they take offence when it is pronounced properly. Just a thought!
Now I be Jolly Roger the Red so I be leanin heavy towards the "No Quarter" Flag and I be called "Old" many a time so if'n ye be thinkin me the devil; Thinks what yer will says I. What say You?