What is the true definition of hooking up?

Really only kissing. That is French kissing or tongue kissing.

A Differing Opinion

The meaning of "Hooking Up" depends very much on the context of the use for the term. For example, "Let's hook up when you get back to town" can be said to mean lets to clubbing or catch a beer at the pub. However, if the two were lovers that would imply a "leg-over" and probably lots of snogging.

It would also depend on the place it was said. In a night club one could easily imply a to have sex but said in a family context it could mean text me later and lets see what is going on for supper plans.

A Differing Opinion

Now I have to look up "leg over" and "snogging".