MissAdventure
No, I'm not familiar with it.
I presume it means used to the training, highly strung, not for hacking around on.
I may be totally wrong, so I'm probably naive in that respect.
No, it means someone riding a horse which is - to put it very simply - too much for them. If you were to put me on one of the top dressage rider's horses, for example, I would most definitely be 'overhorsed'.
And BlueBelle, as I've repeatedly said, while that may happen at the highest levels of dressage, it most certainly doesn't at the lower levels of the sport - such as riding club or Pony Club dressage. There's no 'holding in unnatural poses' or whipping involved there.