THE WORD ``feisty'' appears in the New Shorter Oxford Dictionary, published last year, where it is defined as ``aggressive, excitable, touchy; plucky, spirited''. It is now often applied to women, in the sense ``assertive, self-assured, tough''. The word is being used in more and more contexts (I have recently seen a sports car described as ``feisty''), and in time this will stretch its meaning still further.
Chambers' etymology, while correct, does not tell the whole story; the American ``fist'' comes from a Middle English verb meaning ``to break wind''. Its development is described fully in Questions of English (OUP), a book based on inquiries sent to Oxford Dictionaries by members of the public.