ASA was Brainard's first name, and most people called him Ace. Which is why a lot of people believe he is the origin for the term. Its a talked about in wiki, of course, wiki is 100% authenticate.
I think the Orioles throwing 4 twenty game winners, had 4 aces that season.
To me the ace, is the stopper, the guy, you count on, that will have a good chance to win the game, before he even tosses the first pitch. You got a 1 to 4 game losing streak, you need your Ace to come out and put a end to the Ls.