Green’s Dictionary of Slang

(US) a pro-black civil rights agitator; also attrib.

1865		Dly Phoenix (Columbia, SC) 23 Sept. 2/4: It is well known that the niggerheads of the North are, as public men, the most unreliable in the country.
1868		N.Y. Herald 6 July 3/2: The d——d radical party, with its niggerhead Congress [DA].
1868		N.Y. Herald 7 July 6/2: It demanded [...] a final settlement of all the vexed questions of copperheadism and niggerheadism [DA].
1872		Schele De Vere Americanisms 281: They were Democrats, and retorted upon violent Union men by calling them Niggerheads.
1979		Maledicta III:2 170: nigger-headn [...] 2: Northerners who used violent measures in dealing with the slavery question; term used by Copperheads, by those who wanted war.