Then people who know about Insta said that “throwback” means it’s from before.
You have been very sheltered not to be familiar with the term throwback. I think it has been around a long time, like “that guy’s a throwback to an earlier era, when men were allowed to stab at will”. But it came into more common usage when the NFL began having games with teams wearing “throwback” jerseys, jerseys in the colors and style that their team wore in the 50, 60s or 70s.
Then, when Facebook became popular, there started to be a “thing” called “throwback Thursday” where people would post an old photo of themselves or someone else. I guess they have throwback pictures on Instagram, too, I don’t know, I don’t use it.
“Then people who know about Insta said that “throwback” means it’s from before.”
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I recognized it from my Facebook days. People would post old photos on “Throwback Thursday”. Old photos in general were called throwback photos.