“Hooking up” is a phrasal verb – which means (as all my ESL students know) it is an expression containing a verb and a preposition.
Once upon a time is a phrase we use to start fairy tales. It was also used in the very first Star Wars film, in the opening captions where we learn that the story takes place “Once upon a time, in a galaxy far, far away. Sometimes we use “once upon a time” ironically to mean “a not so long time ago in the past,” as in the following:
Once upon a time “hooking up” was an innocuous expression used in the following way:
John: My flight was delayed, so I won’t be able to meet you at the breakfast. I should get to the conference by noon.
Kevin: That’s fine. There’s a lunch break at noon. We’ll hook up then.
John: Great! I’m looking forward to it.
In this context, “hook up” is very similar to “meet up.” Here’s another example, just to make the point:
Sarah: So did you have a chance to talk to John at the conference?
Kevin: No, his flight was delayed, and then he got stuck in traffic, and I was on the panel in the afternoon. We never managed to hook up.
However, in recent years the expression “hook up” or “hooking up” has taken on another meaning, which has just about supplanted the previous meaning. Here is an example of how you are likely to hear it used:
John: I really like Sarah, but I think she’s dating Kevin isn’t she?
Beth: Dating? I don’t know about that. They may have hooked up a couple of times back in college, but I think now they’re just friends.
John: Friends with benefits?
Beth: Maybe once upon a time. I doubt it in the present. Kevin lives with his girlfriend, and I hear she keeps him on a pretty tight leash. Honestly, I doubt that Sarah and Kevin are more than co-workers.
John: Wow. I don’t think I’d be too comfortable with that history if I was Kevin’s girlfriend!
Beth: Oh c’mon! What happens in college stays in college.