Horsing around

[insider]

How did horses become involved in a term for innocent fun?

[Ice_Wolf]

From Word Detective:

“Horse” has been slang for a rough, unpleasant joke or a bad turn of fortune since at least the late 19th century, and is most often heard in the phrase “horse on,” as in “The final bill, when it arrived, was a horse on the host,” meaning an unpleasant shock. This sense of “horse” is probably related to “horseplay” and “horsing around,” meaning a rough, physical kind of play.

There are other horsey phrases explained and theorised over here.

[j.c]

Horses not only horse around, it’s also true that when they are “horsing around” the mood can turn quickly to real fighting… or to just stopping and grazing. It’s both a playful way to behave, and specifically a playful mood that comes and goes seeming randomly.

[Ice_Wolf]

“Horsing around” does tend to imply a bit of “rough and tumble” in the larking about, though, j.c., rather than the “oh, hey, let’s play races around the paddock a bit before munching more grass”.