I’ve used this term for so many decades that when it came up tonight in conversation, the question arose of where it originated. I thought I knew, but Andrew Ferguson and I decided to do a search.
We could find nothing directly applicable, let alone authoritative, but the question itself kept popping up with no direct answers.
Here, then, is my best recollection.
I was watching a baseball game in the late 1960 or 1970s. a rare enough occurrence for me that it must have been a playoff or World Series game, but I can’t swear it was. It may have been a game between the Mets and the Cards, because I remember the batter wearing red, and the fielders were wearing a dark color; I’m guessing blue, because I wouldn’t bother watching a Yankees game.
The batter, wearing red, got a base hit and slid into first, barely making it safely in a cloud of dust. He stood up, called time, and dropped his trousers to shake out the dirt. (Yes, he was thankfully wearing underwear; I think tight boxers.)
The stadium crowd roared with laughter. You could almost literally hear the off-camera commentators’ jaws drop. and they laughed.
One of them then said something to the effect of, “I think he must have had a brain fart or something. Maybe for a moment he thought he was in batting practice.”
Now, I don’t know if he came up with that on the spot or he’d heard it somewhere previously; maybe the military. But that was absolutely the first time I heard that phrase, so it dates back at least 50 years.

