A defamatory or injurious statement—usu in phr give one a (or the) backcap to backcap v one.
1873 Sun (Logansport IN) 25 Dec /4 (newspaperarchive.com), The Illinoise State grange . . among other resolutions adopted the following: “Resolved, That the election to responsible offices of Brothers who are editors, publishers, or proprietors of public journals, has a tendency to weaken our influence as an organization.” . . Oh! grangers, please don’t shut these fellows out on the first heat. Do you not see how gladly they get down on their abdomens to cater to your desires; whereforely will you give them the back cap thus? 1876 Cincinnati Enquirer (OH) 28 Apr 4/5, George Alfred Townsend . . gives Tilden this amusing “back-cap,” as the boys of the street would put it: “Either Tilden or Davis would make a respectable President. . . But Governor Tilden is not liable to live out his term or to possess the full vigor of administration while exercising it.” 1883 Twain Life on Missip. (Boston) 514, I told him all about my being in prison. . . & now I did n’t fear no one giving me a back cap (exposing his past life) & running me off the job. 1970 Major Dict. Afro-Amer. Slang 22, Backcap: (1940s) a sharp reply—associated with The Dozens.