Green’s Dictionary of Slang

a derog. term for a black person.

1843		Whip & Satirist of NY & Brooklyn (NY) 14 Jan. n.p.: Miss Bagsby [a prostitute] [...] has of late cast day and martin [and] adopted that of white.
1894		Truth (Sydney) 20 May. 1/6: There are ‘Tarpots,’ ‘Charcoals,’ and ‘Day and Martins’ all over the continent.
1900		Bulletin (Sydney) 21 Apr. 24/3: ‘That your hoss over there?’ said a grinning Day and Martin, with a malicious twinkle in his eye, pointing to a beast tied up with others to a fence.