Green’s Dictionary of Slang

1. (US, also big N) a black person [nigger n.¹ (1)].

1970		J. Bouton Ball Four 372: ‘N’ is black code for ‘nigger’.
1971		F.J. Hodges in Black Wisdom 25: If what is said reaches you [...] You know what it means to be an ‘N’.
1976		D. Hall Dock Ellis In the clubhouse they use racial epithets on each other: nigger, Dago. [...] Jerry Reuss [...] was astonished: ‘You use the big N,’ he said. Anybody who said nigger in Houston, he swore, would have got himself killed.

2. pertaining to drugs.