mustang, n. — Green’s Dictionary of Slang

mustang n.

[SE mustang, note US milit. jargon mustang, an officer who has been commissioned from the ranks]

(US black) an independent woman who is ‘hard to ride’.

1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
2012		G. Pelecanos (con. 1972) What It Was 116: Monique had a mean-mustang look to her that Jones liked [...] an untamed country girl.