To horse around means to behave in a silly way, making noise and causing disruption.
During the school holidays there was no peace in the house. Johnny and Annie horsed around all day, every day.
I was horsing around with some friends in the library - we got told to leave.
If someone tells you they heard something from the horse's mouth it means they heard it from the original source; therefore, it must be true.
Interest rates are definitely going up next month. The finance minister was talking about it on TV - I heard it from the horse's mouth.
Horses were domesticated 6,000 years ago on the grasslands of Ukraine, southwest Russia and west Kazakhstan, according to a study by a Cambridge University team.