According to Wikipedia…
Stout is a dark, top-fermented beer with a number of variations, including dry stout, oatmeal stout, milk stout, and imperial stout.
The first known use of the word stout for beer was in a document dated 1677 found in the Egerton Manuscripts, the sense being that a “stout beer” was a strong beer, not a dark beer. The name porter was first used in 1721 to describe a dark brown beer that had been made with roasted malts. Because of the huge popularity of porters, brewers made them in a variety of strengths. The stronger beers, typically 7% or 8% alcohol by volume (ABV), were called “stout porters”, so the history and development of stout and porter are intertwined, and the term stout has become firmly associated with dark beer, rather than just strong beer.