Green’s Dictionary of Slang

1. a derog. form of address to an anonymous Mexican or other Latin man.

1962		T. Southern ‘The Road Out of Axotle’ in Southern (1973) 107: I think you’ve probably picked the wrong crowd this time, Pancho.
2012		J. Díaz This Is How You Lose Her 16: [to a Dominican] Why don’t you beat it, pancho.
2019		S.M. Jones Lives Laid Away [ebook] ‘Thinkin’ about naming a couple holding cells after you and Pancho. The Imbecile Suites’.

2. a Puerto Rican.

1993

A. Rodriguez Spidertown (1994) 14: If he gets away with that, every li’l pancho be dickin’ me up the ass.