At my barn we use it "incorrectly" to tell if we are just riding or if we are lessoning. Ie, I would say "I'm just going to hack on monday" meaning that my trainer doesn't have to work him or be there for a lesson. I know it's not really right (I always thought it was essentially an english trail/road ride) but it's become an easy way for us to get the point across. I'm not sure how other people differentiate between lessoning and just riding.
I'm a big fan of the real version of hacking. I didn't really have trails near me when I grew up (at least not ones that we were supposed to be on
) so I rode on the road a lot as well.