The following was posted last year by Mike Dossey sounds as good as anything I have seen on our Dossey origin:
If you take your question literally, the easy answer is that it is English, most commonly found in the Sheffied/Yorkshire area. But what did it come from? It is used interchangeably with Dorsey (or D'Arcy), also found prominent in the Yorkshire area. Some of the early Virginia/Maryland settlers used the Dossey spelling. And if you look in the London Society of Genealogists Library, you will find Dossey references but no Dorsey references. Bottom line is that the Dossey spelling is an old English version of names that include D'Arcy (thus Norman or French), O'Dorchaide (Irish), or Doxey/Dockseys, likely emanating from the place name Doxey in Staffordshire (eg Richard de Dokeseye, on the rolls in 1298). As far as one family's preference to take the Dorsey spelling versus another to take the Dossey spelling, there is various family lore on that, possibly including one's views on slavery or religion (if not simply the preference of the census taker). Clear as mud?