Suddenly and, sometimes, unexpectedly. For example, The alarm went off, just like that, or And then they walked out, just like that.
Also, like that. Very friendly or intimate with one another. For example, Bill and his boss often see each other socially; they are just like that, or Joe and Jane are always together; they’re like that. This expression is usually emphasized by the speaker’s holding up two fingers and either keeping them together or crossing them to show the closeness or intimacies of the parties being discussed. [Colloquial; early 1900s]