Green’s Dictionary of Slang

1. (US) an (affectionate) term of address.

1830		E. Temple Travel in [...] Peru II 444: ‘Come along, amigo,’ said I, and away we went.
1897		A.H. Lewis Wolfville 12: ‘The bridle’s plumb off to you, amigo,’ says Cherokee, an’ his tones is some hard.
1907		W.M. Raine Bucky O’Connor (1910) 101: We’re going to laugh and talk as if we were the best of friends, but my hand ain’t straying any from the end of my gun. Get that, amigo?
1931		R.E. Howard ‘Texas Fists’ Fight Stories May 🌐 How about it, amigo? Will you mosey back up in the hills with us and flatten this big false alarm?
1938		R. Chandler ‘The King in Yellow’ in Spanish Blood (1946) 92: It’s a little early, amigo.
1949		R. Chandler Little Sister 161: Nothing I could discuss on this telephone, amigo.
1959		A. Zugsmith Beat Generation 6: You made it quick, amigo.
1967		P. Thomas Down These Mean Streets (1970) 17: ‘Cool it, man,’ I said and grinned a screw-you-amigo smile.
1970		G. Scott-Heron Vulture (1996) 111: Okay, amigo, I give up.
1981		J. Ellroy Brown’s Requiem 129: You’ve got that all wrong, amigo.
1990		S. Morgan Homeboy 169: Amigo, look like Dillinger or Daffy Duck.
2000		G. Pelecanos Shame the Devil 24: You listen to the news, amigo?

2. a friend.

1998		(con. 1986) G. Pelecanos Sweet Forever 11: His dust days were over, though; he’d lost too many amigos to that stuff.
2005		K. Huff A Steady rain I i: Fifty guys [...] who just all happen to be a lot more ethnic than me and my bog-hopping amigo paisan over here.