A “party pooper” is someone who leaves a party early, or who declines to participate in or to enjoy the party. In politics, a “party pooper” is someone who doesn’t agree with his or her own political party or group.
 
“Anyone still in possession of his own heart tomorrow morning is a party pooper” was printed in the Daily Northwestern (Northwestern University, Evanston, IL) on February 14, 1945.
 
     
Wiktionary: party pooper
Noun
party pooper (plural party poopers)
1. (informal) One who dampens fun; especially, one who declines to participate in or enjoy a party or some other group activity; one who does not agree with the opinions of a group
 
(Oxford English Dictionary)
party pooper  n. a person who spoils the social enjoyment of others, typically by being gloomy; a killjoy.
1947   Coe College (Cedar Rapids, Iowa) Cosmos 26 Mar. 2/4   It is awful embarrassing to have to be a party-pooper and blush to see such goings on in public.
1951   Newsweek 8 Oct. 28/1   Party pooper has taken the place of wallflower or wet blanket.
         
14 February 1945, Daily Northwestern (Northwestern University, Evanston, IL), “Guess what day this is…or, whose little girl are you?,” pg. 2, col. 1:
Anyone still in possession of his own heart tomorrow morning is a party pooper.
   
Newspapers.com
15 April 1948, Crystal Lake (IL) Herald, pg. 2, col. 5:
Advice: - Dear Upset: -
Let’s face it! Your relatives are party poopers!
 
2 October 1948, Columbia (SC) Record, “Around Town” with Kitty Kaye, pg. 2-B, col. 5:
With the rain and the not-too-good prospects for an exciting game, many party poopers stayed at home and now wish they had gone.
 
7 August 1949, Chicago Sunday Tribune, “Party Is Only as Much Fun as Guests Make It” by Sheila John Daly, pt. 7, pg. 7, col. 4:
Summer is the party season, and unless you’re a social schmoe, you’ll be asked to many of them before vacation is over. Parties are fun, but only as much fun as the individual guests make them, so unless you want to be dubbed a party pooper by your friends, why not cast an eye over these rules designed to make you an ideal guest in the house, instead of a pest in the house?
   
OCLC WorldCat record
Party Pooper? - A decade after a group of moderate-to-conservative Democrats set out to pull their party in a new direction, the Democratic Leadership Council is facing its greatest challenges. Rccent successes have been matched with setbacks, and the future looks uncertain.
Author: Eliza Newlin Carney
Publisher: Washington, D.C., National Journal Group Inc.
Edition/Format: Article Article : English
Publication: National journal. 28, no. 14, (1996): 767
 
Urban Dictionary
party pooper
A person who ruins a party by either stopping the fun or not participating in a certain activity.
Jeff is a party pooper because he won’t play Spin The Bottle.
by Special July 18, 2004
 
Twitter
el laurence: #FBPE
@ajhanlaurence
Replying to @SamInFleet
I think the “poop” in party pooper means deflate (by tiredness), rather than to defecate!
2:59 PM · Aug 2, 2019·Twitter for Android