Why does a horse and the activity of "playing horse" describe one who is teasing, ridiculing, or even flirting? The survey of College Words and Phrases by Eugene H. Babbitt published in 1900 lists the expression to play horse with and the many meanings of it including to tease, to ridicule, and to flirt. I'm curious to know if anyone has any ideas as to how "playing horse with" became associated with these meanings?
Not sure, but may be it originated from the way young horse's frolic and play-fight.Its just silliness and fun with no constructive purpose. So, perhaps it was thought that anything a "fool" was engaged in was similar to "horse play" and was frivolous, And hence the association.