Green’s Dictionary of Slang

slandering, talking maliciously behind someone’s back; also as adj.

1880		Northern Tribune (Cheboygan, MI) 30 Oct. 1/3: Their only stock in trade, during the campaign, has been ‘mud-slinging’.
1884		Wkly Graphic (Kirskville, MO) 31 Oct. 2/2: The mud-slinging menagerie which has been [...] inflicting its stench on the good people [etc.].
1889		Great Falls (MT) 20 Nov. 1/4: One wonders what a real, old-fashioned mud-slinging campaign will be like.
1892		Mohave Co. Miner (AZ) 22 Oct 2/3: Mudlsinging will not win votes.
1899		Custer County Republican (NE) 10 Aug. 4/4: It’s mud-slinging on the part of a republican to question your devotion to principle.
1908		Essex Newsman 7 Mar. 3/8: Mr J. Ramsay Macdonald, MP, said [...] they saw a good deal of mudslinging.
1910		Bulletin (Sydney) 15 Sept. 7/3: The Bench, on this occasion, briskly checked the Gentleman One in the midst of his mud-slinging.
1922		Joyce Ulysses 603: And then seventytwo of his trusty henchmen rounding on him with mutual mudslinging.
1926		L.C. Wimberly ‘American Political Cant’ in AS II:3 135: Our obstreperous, ‘mud-slinging’ histrionics of State.
1936		Western Morn. News 24 Oct. 7/4: Indignation at what he termed ‘mud-slinging’ was expressed.
1945		Birmingham Dly Gaz. 19 June 4/3: [headline] No ‘Mudslinging’.
1953		R. Chandler Long Good-Bye 140: No trial, no sensational headlines, no mud-slinging just to sell newspapers without the slightest regard for truth or fair play or for the feelings of innocent people.
1973		Stage (London) 25 Jan. 12/2: It is important tnat people conducting discussions should take care that one side or the other is not exposed to mud-slinging.
1980		Times (Shreveport, LA) 31 Oct. 7/3: Most of them feel ‘mudslinging in campaigns is immature [...] and not worthwhile’.
1985		B. Humphries Traveller’s Tool 21: That’s just part of a carefully orchestrated shit-slinging campaign.
1998		Age (Melbourne) 16 Sept. 19/2: Mud-slinging can put voters off.
2006		Desert Sun (Palm Springs, CA) 26 Oct. 70/3: The election in Cathedral City was free of gros mudslinging.
2012		Houston Herald (MS) 1 Nov. 326/1: Mudslinging sometimes turns out to be a way of findingout important aspects of a candidate’s character.