What it means to hold your horses

All this time I've been grabbing my balls. Smh

31 here. Had no idea about the stable. Thought it meant to rein yourself in.

I've heard "hold your horses and keep em still"

Who da fuq is making this shit up? "Hold your horses" has nothing to do with "stables". It's literally "don't act impulsively", because a nervous horse would bolt and run away (and cause grievous damage, for which the owner would be held accountable), so their owners would have to hold them by the reins.