Green’s Dictionary of Slang

1. the penis.

1966		Trimble 5000 Adult Sex Words and Phrases.
1972		R.A. Wilson Playboy’s Book of Forbidden Words.

2. (US black) an affectionate or familiar term of address to a younger man or child.

1978		W.D. Myers It Ain’t All for Nothin 52: I [i.e. a 12-year-old boy] went outside to the bathroom. I said, ‘Good morning,’ and Stone said, ‘Good morning, little brother’.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 7: Look here, little brother.