(alternate spelling of hardon)
1. An erection
2. A problem
Although definition 1 is far more common, 2 is occasionally used. It is used with "for", as in "Having a hard-on for..." It implies an unhealthy fixation, and it is pejorative, in that (obviously) it implies a homoerotic obsession.
Ever since I got Jim fired, he's had a real hard-on for me.
Stop picking on him! Why do you have such a hard-on for him anyway, dude?