So there was this girl and her horse would not stop following her, so she said, "Stop horsing around!"
Get it? "Horse-ing."
Alright listen up, buttercup. This 'girl' tells her horse to quit 'horsing around'. Get it? It's a play on words, see? 'Horsing around' means to act wild and playful, but it ALSO sounds like she's telling a horse to quit it. Boom! Comedy! Bet you didn't know that horses were domesticated around 4000 BC. That's like, a long time ago, even for you.