Other forms: horses around; horsing around; horsed around
When you horse around with your friends, you play in a rough and silly way, wrestling and goofing around.
When people horse around, they're silly and boisterous, fooling around in a physical way. Little kids horse around often, for example. The phrase doesn't immediately make sense, because you don't usually see horses playing in this way. Horse around probably comes from horseplay, and it in turn came from the old-fashioned verb horse, which was once used to mean "play crazy jokes on." Experts aren't sure how it came into use, or what horses have to do with it.