Green’s Dictionary of Slang

feisty adj.

also fisty
[SE fist, a small dog; thus having the characteristics of such a yappy, snappy, energetic creature; the dog shares an ety. with 15C fist, a foul smell, the breaking of wind, but whether, as Wentworth & Flexner suggest in Dict. American Slang (1960, 1975), the dog was so named because one’s own smells could be blamed upon it remains debatable. Equally feasible is the 19C suggestion that such dogs were not much bigger than a man’s fist. Note dial. feist, to strut about, to flirt or show off]

1. [late 19C+] (orig. US) truculent, irascible, impertinent.

2. [20C+] (orig. US) of a young woman, flirtatious (to a greater extent than the speaker sees as proper), showing off, putting on airs, of dubious morality; 1980s+ use tends to perceive this in a more positive light.