Coney Island Butter (mustard)

“Coney Island butter” is another term for “mustard,” a condiment used on the popular Coney Island hot dogs. There is only one known print citation for “Coney Island butter” (in 1947), but it probably was a minor slang term in New York City at that time.
   
   
Google Books
American Notes & Queries
Volume 7
1947
Pg. 86:
“CONEY ISLAND BUTTER”: mustard; term overheard in New York rathskeller.
 
Google Books
Dictionary of American Regional English, Vol. I, A-C
Frederic G. Cassidy, Chief Editor
Cambridge, MA: The Belknap Press of Harvard University Press
1985
Pg. 749:
Coney Island butter n
1947 American Notes & Queries 7.86 NY, “Coney Island Butter”: mustard, term overheard in New York rathskeller.