“County Attorney” and “District Attorney” are names that were applied to “son-of-a-gun stew.” Attorneys weren’t held in high regard by the cowboys who made and ate the stew that also went by the name of “son-of-a-bitch stew.” The “attorney” names date from at least the early 1930s, when laws and lawyers had long since closed much of the range.
   
 
(Dictionary of American Regional English)
county attorney n
=son-of-a-bitch stew.
1933 AmSp 8.1.27 nwTX, County attorney or son-of-a-gun...The term county attorney used as the name of a dish called many unmentionable names is an interesting sidelight on the general opinion of that official.
1977 Watts Dict. Old West 308, Son-of-a-bitch stew...included sweetbreads, marrow gut, and kidneys added to the best meat of the calf…Also, in the later days, known among cattlemen as district attorney...and county attorney.
 
(Dictionary of American Regional English)
district attorney n Cf county attorney
=son-of-a-bitch stew.
1944 Adams Western Words 50, District attorney—Another name for the son-of-a-bitch stew. When the law began its westward march and started to question and clamp down on the cowboy government of the happy, carefree days, the blame for this cramping of liberties was placed upon the lawyers. This caused the riders of the range to feel somewhat resentful toward the law, and soon they began calling this dish district attorney. 
 
17 January 1932, Dallas Morning News, “Stewin’ a Son-of-a-Gun,” section IV, pg. 2:
Those early days on the range and trail were happy and carefree for most, but I am told that when the new law began its march westward, began to question and to clamp down on cowboy government, that lawyers did more than a little to cramp liberties. This caused the riders of the range to feel somewhat resentful toward them and soon cowboys no longers called that stew by its old name—instead, dubbed it “District Attorney.”
 
Why? Ask an old-time cowboy.
 
Google Books
Cowboy Lingo
by Ramon F. Adams
Boston: Houghton Mifflin Books
2000 (1936 original printing)
Pg. 145:
“Son-of-a-gun stew,” a favorite dish of the cowboy, was made of brains, sweetbreads, and choice pieces of a freshly killed calf. If the cowboy wished to be polite he called it by this name, but if no delicate ears were present, he called it by its true name, something that has always been a fighting word. When the law began its march westward and started to question and to clamp down on cowboy government of the happy and carefree early days, the blame for this cramping of liberties was placed on lawyers. This caused the riders of the range to feel somewhat resentful toward the law and soon they began also calling this dish “District Attorney.” The (Pg. 146—ed.) implication was obvious. In making this stew, the cook was said to “throw everything in the pot but the horns and hide.”
   
Google Books
The American Thesaurus of Slang
by Melvin van den Bark and Lester V. Berrey
New York: Crowell
1953
Pg. 847:
county or district attorney, son-of-a-gun (stew), a dish made of “choice” pieces of a calf, which consists of “everything but the hair, horns and holler”
 
Google Books
The Humor of the American Cowboy
by Wright Morris
Lincoln, NE: University of Nebraska Press
1980
Pg. 74:
In fact, in some sections the name “son-of-a-gun stew” was changed to “district attorney stew.”