I am 80% sure this story is by Sturgeon, but I can't find it (his titles are not very helpful).
The protagonist has a brother or frenemy who is a cadet at a space academy. When the space cadet comes home, he talks constantly in space academy slang(*) ("cool your jets, there, buddy!") to constantly remind everyone he is a space cadet and how cool he is. He steals the protagonist's girlfriend because of his awesome cool factor.
It maybe turns out that he washed out of the academy, or was a janitor there, or something like that, and was just trying to impress everyone back home.
I can't remember if this space cadet guy is the focus of the story or just a side character. He reminds me a lot of some people on social media, which is why I'm trying to find the story.
Does anyone remember this story?
(*) What Sturgeon thought space academy slang would be when he was writing this in the 40s or 50s