Douglas Harper's Etymology Dictionary
"courageous, masculine," 1959, first attested in Norman Mailer (writing of Truman Capote); see balls + -y (2).
Wiktionary
a. (context vulgar slang English) tough and courageous; having balls.
Usage examples of "ballsy".
But it's a ballsy fight-scene up there on the stage having been intricately choreographed by an Oriental guy Himself rented from some commercial studio and put up in the HmH, who ate like a bird and smiled very politely all the time and didn't have even a word to say to anybody, it seemed, except Avril, to whom the Oriental choreographer had cottoned right off balletic and full of compelling little cornerings and near-misses and reversals, and the theater's audience is rapt and clearly entertained to the gills, because they keep spontaneously applauding, as much maybe for the film's play's choreography as anything else which would make it more like spontaneously meta-applauding, Hal supposes because the whole fight-scene has to be ingeniously choreographed so that both combatants have their respectively scaly and cream-complected backs155 to the audience, for obvious reasons .
He alights next to me, saying, "Jack, that was the ballsiest half of an interview I ever heard.
Yet at the same time, Luigi was right, he had done his best, right from the start when they’d all walked into City Hall in the ballsiest escapade this side of the Trojan horse.
He'd have been Jackson's first choice, anyway: he looked ballsy, alert and fit as a butcher's dog.