Chuck Eater

A “chuck eater” is another cowboy term for a “greenhorn” or “pilgrim” or “tenderfoot.” To the old hands, a “chuck eater” can’t do the work and is only good at eating the chuck (chow).
 
   
Google Books
Cowboy Lingo
by Ramon F. Adams
New York: Houghton Mifflin Books
2000
1936 (original copyright)
Pg. 239 (Index):
Chuck-eater, 26.
 
Google Books
Dictionary of the American West:
Over 5,000 Terms and Expressions from Aarigaa! to Zopilote
by Win Blevins
Seattle, WA: Sasquatch Books
2001
Pg. 84:
chuck eater (a greenhorn learning to cowboy, but only effective at doing away with the chuck).
   
24 September 1927, Daily Northwestern (Oshkosh, WI), pg. 2, col. 3:
DIALOGUE OF COWBOYS
GREEK TO STRANGERS
 
San Angelo, Tex.—(AP)—A dictionary would be about as worthless as a song in a hurricane to a New Yorker trying to find his way around the ranch country of the west.
 
Cowboyese, the dialect of the ranges, is as intricate and snappy as New Yorkese and changes almost as rapidly. Some of the terms used in the pioneer days have come down unchanged through the years, but other influences—mainly that of the cavalry in which most of the cowhands fought in the world war—are apparent in the dialect.
 
What would a native of New York’s East Side do if confronted with a conversation like this:
 
“The top screw mounted his cutting horse, and, followed by a group of chuck eaters, started to trail a bunch of cattle. The corral rope was on his saddle, next to the sougan, and as he placed a brain tablet in his mouth, his mount began to swallow its head and soon turned the pack.”
(...)
“Chuck eater” is the name applied to the young man from the east who comes out to learn the game.