A shortened version of bad ass/badass. It is also a "cleaner" alternative to using the real word, like if you're around kids or something. Used less often when referring to a person though (as is the case with "bad ass"), more often used to describe object, events, etc. Also IM/text shorthand for bad ass.
"Dude that shirt is bad a!"
Cindi: "Are you going to the concert?"
Rick: "Yeah, I hear it's supposed to be totally bad a."