Knuckle Sandwich (punch in the mouth)

A “knuckle sandwich” is, literally, a sandwich of knuckles (such a pig knuckles). The slang meaning of “knuckle sandwich” is a punch to someone’s face. Someone’s knuckles hit another’s mouth.
 
“Knuckle sandwich” was printed in the San Francisco (CA) Chronicle on August 19, 1940.
 
       
Wiktionary: knuckle sandwich
Etymology
The term describes a person’s fist connecting with another person’s mouth, as if the latter were eating a sandwich.
Noun
knuckle sandwich (plural knuckle sandwiches)
1. (slang) A punch to the face, especially to the mouth.
 
(Oxford English Dictionary)
knuckle sandwich  n. slang a punch in the mouth.
1973   A. Buzo Norm & Ahmed 12   He tried to hang one on me at Leichhardt Oval once, so I administered a knuckle sandwich to him.
1973   Ottawa Jrnl. 17 July 23/3   Give the guy a knuckle sandwich and let the teeth fall where they may.
   
19 August 1940, San Francisco (CA) Chronicle, pg. 4H, col. 3 photo caption:
HOW’D YA LIKE A KNUCKLE SANDWICH, PARD?
 
30 November 1943, Hammond (IN) Times, “Speculating in Sports” by John Whitaker, pg. 9, col. 2:
“The senior Miller says that on at least ‘four or five occasions’ Crighton received a ‘knuckle sandwich’ from some of the other fellow, and in full view of one or more of the officials, yet nothing was done about it.
 
23 February 1945, The Journal-News (Nyack, NY), “Rounding Off on Sports” by George Mueller, pg. 7, col. 3:
The sergeant, who was a professional boxer, snapped: “Say that again, Fisher, and I’ll give you a knuckle sandwich.”
 
OCLC WorldCat record
Knuckle sandwich : growing up in the working-class city
Author: David Robins; Philip Cohen
Publisher: Harmondsworth ; New York : Penguin, 1978.
Series: Pelican books.
Edition/Format:   eBook : Document : English
     
Google Books
Contemporary American Slang
By Richard A. Spears
Lincolnwood, IL: National Textbook Co.
1991
Pg. 124:
knuckle sandwich n. a blow struck in the teeth or mouth.
 
Urban Dictionary
knuckle sandwich
to administer a mouthful of fist; to punch someone in the face
you want a knuckle sandwich?
by Branid January 03, 2004
 
Google Books
The Art of the Knuckle Sandwich: Learning to Get Back Up When Life Knocks You Down
Ricky M. Hansen, Jr.
Createspace Independent Pub, 2013 - Self-Help - 124 pages