All that is often an informal synonym of very or truly, especially in negative statements such as, “He is not all that great at tennis.” From this statement we can infer that “he” is supposedly a good tennis player and that the speaker disagrees with that belief.
In U.S. English, meanwhile, all that functions informally as a standalone predicate adjective—e.g., “People think he’s a great player, but I don’t think he’s all that.” That doesn’t even need an antecedent—e.g., “There’s her new boyfriend. He ain’t all that.” In this statement, that doesn’t refer to anything in particular; all that simply means great or impressive.
This latter sense of all that seems to have originated in African-American English during the 1980s. The earliest example noted in the Oxford English Dictionary—which lists this sense of all that as “to be great; to be particularly impressive or attractive”—is from 1989. Funnily enough, our own historical searches using Google’s various tools uncover several instances also from 1989 and none from earlier. That must be the year in which the phrase caught on broadly.
Of course, all that has several other uses. It can mean things related to that or things like that—e.g., “I don’t know much about science and all that.” It can mean all that was referred to or mentioned before—e.g., “Goodbye to all that.” And when all is a pronoun in its own right, that can become the subject of a restrictive adjective clause modifying all—e.g., “All that glitters is gold.”
In these sentences, all that means very or truly:
There’s a saying that common sense isn’t all that common. [Newsday]
All told, Ohio is not all that different socio-economically from America as a whole. [Financial Times]
If I was John Kirwan, I wouldn’t be all that worried about what many people see as an overly young Blues side. [New Zealand Herald]
And in the following examples, that has no antecedent, and all that simply means great or all that he, she, or it is made out to be:
Among them is the beautiful, classically trained, mean girl, the horribly annoying Latina from NYC who thinks she’s all that and a 4-foot-9 Brazilian guy. [New York Post]
Florida’s solid 14-6 triumph over LSU in The Swamp on Saturday proved the Tigers ain’t all that anymore. [London Free Press]
And does the computer-coding genius Zuckerberg come off as arrogant and self-obsessed? Yes. But no more than any sophomore in college who thinks he’s all that. [Cleveland Plain Dealer]