an expression showing regret used in two different occasions: Either when simple plans do not work out, or when the universe cosmically fucks you over. Vonnegut stole it from some french author, and popularized it in Slaughterhouse Five.
1.
"did you meet up with those kids?"
"No, they went home."
"ah, so it goes"
2.
"So you avoidedhitting a deer while driving only to run into a tree?"
"Yes."
"So it goes."