Green’s Dictionary of Slang

Tío Taco n.

also Tío Tomás
[Mex. tío, uncle + taco, the foodstuff; ult. a play on Uncle Tom n. (1)]

(US) a derog. term for a Mexican who is considered insufficiently nationalistic by others.

[US]L.A. Times West mag. 26 Jan. 9/1: The new Chicano [...] doesn’t carry a gun. But he’s not Tio Taco either.
[US]Atlantic Monthly June 45: It was a system that placed high premium on the Tio Taco, or Uncle Tom.
[US]Maledicta VII 24: The Chicanos adapted the blacks’ ingroup slur, Uncle Tom, for a Spanish cornmeal version, Tío Taco, which serves alongside the more parallel Tío Tomás.
[US]H. Rawson Dict. of Invective (1991) 229: Tio Taco (a Mexican Uncle Tom).
Kenosha News (WI) 21 Oct. 11/2A Tio Taco is a boot licker: .
[US]L.A. Times 16 May A11/4: The working-class cousins [...] ridiculed them as uppity and ‘Tio Tacos,’ the Chicano version of Uncle Tom.