An opinion that is not well thought out (with the implication that one has not even taken the time to get off one's horse to give the issue more careful consideration). Now that I've learned more about the cheating scandal and had time to think about it all, I wish I could take back my initial horseback opinions.

To be so totally and utterly confused as to be unaware of one's actions. I've had such a long day that I don't know if I'm afoot or on horseback—I think I need to lie down! I finally got Billy a math tutor because his test grades clearly showed that he didn't know if he was afoot or on horseback in that class.

One who has become unscrupulous or irresponsible due to the sudden acquisition of wealth. The phrase alludes to several related proverbs, such as "give a beggar a horse and he'll ride it to death" and "set a beggar on horseback and he'll ride to the devil," which suggest that an unexpected windfall is often misused or squandered. I'm not surprised to hear that he's broke again—he always seemed like a beggar on horseback.

(By, while, or via) riding on a horse. We'll have to travel on horseback through the mountains. They provide a beautiful tour of the area on horseback.

If a poor person suddenly comes into wealth, they will inevitably use it in unscrupulous or irresponsible ways. It was a heartwarming story to hear about the homeless man who found a winning lottery ticket for nearly $2 million. But just a year later, he's back in the headlines for being back on the streets, having squandered all that money on drugs, alcohol, and expensive super cars. Truly, you set a beggar on horseback, and he'll ride to the devil.