Road Apple

A “horse apple” is horse manure. It’s called a “road apple” when it’s found in the road.
 
”Road apples, horse dung” was cited in The American Thesaurus of Slang (1942). A forgotten theatrical definition of “road apple—a player on tour” was published in the syndicated newspaper column “In Manhattan” by George Tucker in April 1940, as taken from The Theatre Handbook (1940) by Bernard Sobel.
 
“Big Road Apple” (Big Apple + road apple) is a derogatory nickname for New York City that has been infrequently used.
 
   
Wiktionary: road apple
Etymology
Probably with reference to the supposed similarity to rotten apples.
Noun
road apple (plural road apples)
1. (idiomatic, dated, often pluralized) Horse manure, especially when deposited on a road.
 
(Oxford English Dictionary)
road apples  n.  [perhaps compare German Pferdeapfel, lit. ‘horse apple’, in the same sense (18th cent. or earlier)] N. Amer. slang horse droppings.
1923   Chicago Defender 6 Jan. 6/6   We had a notion to take a shot at a few road apples ourself, but they had no tents large enough for our manly figure.
1942   L. V. Berrey & M. Van den Bark Amer. Thes. Slang §124/2   Road apples, horse dung.
1951   ‘M. Spillane’ One Lonely Night v. 112   Smart? Sure, just like road apples that happen behind horses.
1970   J. H. Gray Boy from Winnipeg 53   The best pucks were always those supplied by passing horses, ‘road apples’ we called them.
         
8 April 1940, Lexington (KY) Herald, “In Manhattan” by George Tucker, pg. 4, col. 4:
NEW YORK, April 7—Rubes, road apples and heel beaters…here we go on a Broadway Lexicon.
(...)
road apple—a player on tour.
   
Google Books
Renaissance Dramatic Bawdy (exclusive of Shakespeare):
An annotated glossary and critical essays, Volume 2
BY James T. Henke
Salzburg, Austria: Institut für englische Sprache und Literatur, Universität Salzburg
1974
Pg. 183:
Also, “horse-plum” may = horse feces, horse turd. Cf. the modern “horse apple” and “road apple” = horse feces (Wentworth).
 
17 January 1975, The Post-Standard (Syracuse, NY), “Help!,” pg. 2, col. 1:
Q. What are horse apples and where can they be bought? G.L.W., Syracuse.
A. Carol Moeller of Prof. Pauline Atherton’s Syracuse University class in bibliography and information service says, “Anyone who grew up on a farm can tell you that horse apples is a slang term for horse manure. The American Thesaurus of Slang lists other words, e.g., bisquits, road apples. Contact any local riding stable for acquisition.”

Google Books
The Great Santini
By Pat Conroy
Boston, MA: Houghton Mifflin
1976
Pg. 102:
“Well, you start boiling grits in a pot. Then you go down by the highway and get some horse turds or as you civilized southerners call them ‘road apples.’ Well, you take the road apples and dump them into the grits. You boil the combination for fifteen minutes. No more. No less. When you’re done, you pour the grits down the drain and eat the road apples.”
 
OCLC WorldCat record
Road apples & Meadow muffins
Author: Susie Killman; Hildred Goodwine Phillips
Publisher: [U.S.] : [The author], ©1994.
Edition/Format:   Print book : Poetry : English
 
Urban Dictionary
road apples
horse shit on the road
by crap magnet May 25, 2003
 
OCLC WorldCat record
The Road to Heaven Is Paved in Road Apples The Memories of a Horseman.
Author: Hunt, Bill.
Publisher: Authorhouse 2014.
Edition/Format:   Print book : English