“The last one in (the water) is a rotten egg!” is what some children say just before swimming. A “rotten egg”—meaning a worthless or contemptible person—is old slang.
   
“The last one over (across the road—ed.) is a rotten egg” was cited in print in 1896. “Last one out (of a hole—ed.) is a rotten egg” was cited in 1904. “The last man in (the swimming hole—ed.) is a rotten egg” was cited in 1921.
 
     
(Oxford English Dictionary)
rotten egg, n.
fig. A worthless or contemptible person or thing. Cf. egg n. 4a.
1686   J. Bunyan Bk. for Boys & Girls 8   The Hyppocrite, sin has him in Possession, He is a rotten Egg under Profession.
1884   Pet Stock Pigeon & Poultry Bull. Jan. 132/2   If..we can’t afford to denounce rascals and suppress cranks, then any number of brawls and exposures won’t hurt us any, for the very excellent and aged reason, that ‘it’s hard to work to spoil a rotten egg’.
1898   A. B. Gomme Trad. Games II. 403 (heading)  After the tug the side which has lost is called ‘Rotten eggs, rotten eggs’.
2004   M. S. Clark Quarter for Kiss xxiv. 163   ‘Last one in’s a rotten egg!’ Tom said, pulling off his T-shirt and diving from the bow.
 
Chronicling America
27 February 1896, Democratic Northwest (Napoleon, OH), pg. 6, col. 4:
Joy of Being In the Majority.
“The last one over is a rotten egg,” said the little girl, and all the children ran across the road—all that is, except the boy who would have been last. He sauntered away, saying, “I’m not in it.” But he was in it, for they all yelled: “George is a rotten egg! George is a rotten egg!” and got much delight therefrom, while I meditated on the joys that, even at this early age, come from being in the majority.—Chicago Record.
   
7 May 1904, The Patriot (Harrisburg, PA), “Hole a Prison for Three Youngsters: They Jumped Into a Sewer Excavation Playing Follow the Leader,” pg. 1, col. 4:
“Last one out is a rotten egg,” he exclaimed, as he scrambled on top of a dirt heap and threw clods of earth at his companions.
 
28 July 1921, Abilene (TX) Reporter, “A Pleasant Time Was Had by All,” pg. 3, col 1:
“Let’s all go in, boys,” said Judge Stinson. “We’ll go in after the fashion that we did when we used to go in the old swimming hole. We started running toward the hole, undressing as we run and throwing our clothes behind us, and the last man in is a rotten egg.”
 
7 July 1935, Charlotte (NC) Observer, “The Sleeping Log,” Observer Junior, pg. 3, col. 4:
“Gee! This is a swell day for swimming, ain’t it, Jim?” exclaimed Pete as they drew near the pond. “Let’s go out on that log and dive in the deep part. Last one in is a rotten egg!”
 
11 May 1938, Corpus Christi (TX) Times, “Survey Made by North Beach Swimming Instructor Shows More Than Half of Schoolchildren in City Cannot Swim,” pg. 7-B, col. 2:
It doesn’t speak so highly for juveniles whose fathers probably remember a creek or river near some midwestern town that echoed to the yell of “the last one in is a rotten egg” about this time of the year
 
8 July 1938, Cleveland (OH) Plain Dealer, “Back to Old Simming Hole” by Oscar A. Bergman, pg. 12, col. 7:
“Hey, Skinney, let’s go swimmin’...the last one in is a rotten egg.” Those were the good old days back at the ol’ swimmin’ hole when all we wore were our birthday suits…remember?
 
OCLC WorldCat record
Last one in is a rotten egg,
Author: Leonard P Kessler
Publisher: New York, Harper & Row [1969]
Series: Sports I can read book.
Edition/Format:   Print book : Fiction : Juvenile audience : English
 
OCLC WorldCat record
Last one in is a rotten egg!
Author: Diane De Groat
Publisher: New York : HarperCollins, ©2007.
Edition/Format:   Print book : Fiction : Primary school : English : 1st ed.
 
OCLC WorldCat record
The last one is a rotten egg
Author: Tina Gagliardi; Patrick Girouard
Publisher: Ednia, MN : Magic Wagon, 2009.
Edition/Format:   eBook : Document : Fiction : Primary school : English