Using the phrase "my sweet summer child" violates like half of the comment guidelines and is a boring canned phrase to-boot. If you want to insult someone, at least be creative about it.
"Sweet summer child" actually is a kinda interesting insult—it was actually coined by Game of Thrones, and yet has so thoroughly spread that most people don't realize that. I certainly was shocked when I learned that, because I have never even read or watched the series, and so the entire in-universe meaning of it was lost on me—and yet from usage, it was incredibly clear what it meant. And it's catchy, to boot.
(There were some sporadic uses before Game of Thrones, but we're talking like 100 years before Game of Thrones, and the meaning was a bit different.)
Sweet summer child, it's been more than sporadically used in the American South long before Game of Thrones, and well within this century. GRR Martin just used what was already a well known phrase, and maybe the show made it trendy but he didn't "coin" it in any meaningful sense.
Do you have documented evidence for that? That’s the usual reaction (and mine, too, given my unfamiliarity with GoT), but every discussion around this turns up a few hits from the 1800s, and then basically nothing until GoT came out. Or maybe my info is out of date?