“Skunk eggs” were what cowboys called onions. The term “skunk egg” appears in many collections of cowboy lingo after 1950, but pre-1950 citations are rare.
   
   
10 May 1943, Ogden (UT) Standard-Examiner, pg. 5, col. 1:
Onions (skunk eggs, as Mr. Ford calls ‘em) will be ready to harvest soon.
   
Roundup Recipes
by Bonnie & Ed Peplow
with the help of the Arizona Cowbelles
Cleveland, OH: The World Publishing Company
1951
Pg. 270 (Glossary):
SKUNK EGGS: Onions.
 
14 September 1952, New York (NY) Times, pg. BR10:
(Book review of Come an’ Get It: The Story of the Old Cowboy Cook by Ramon F. Adams—ed.)
It was all meat; the only vegetable ever admitted to a sonofagun was an occasional onion which the cowboy called a “skunk egg.”
     
Google Books
Dictionary of American Slang
by Harold Wentworth and Stuart Berg Flexner
2nd supplemented edition
New York, NY: Crowell
1975
Pg. 483:
skunk egg
An onion, still some Western and Southern rural dial. use.
 
Google Books
The Dictionary of American Food and Drink
by John F. Mariani
New Haven, CT: Ticknor & Fields
1983
Pg. 368:
skunk egg. Cowboy term for an onion.
   
Google Books
The Cowboy Encyclopedia
by Richard W. Slatta
W. W. Norton & Company
1994
Pg. 354:
SKUNK EGGS
Descriptive colloquialism for onions.
 
Google Books
Webster’s New World Dictionary of Culinary Arts
by Steven Labensky, Gaye G. Ingram, and Sarah R. Labensky
Upper Saddle River, NJ: Prentice Hall
1997
Pg. 424:
skunk egg Cowboy slang for an onion.