Acronym for "cut a long story short." Notice the irony: It actually shortens the phrase used to shorten an otherwise long story.
Guy: So yeah, she and I were dating, but to CALSS, we broke up because she likes Justin Bieber.
Girl: Hmm, that makes sense. Wait, what's "CALSS"?
Guy: "cut a long story short."
Girl: OMG, that acronym is awesome in so many levels! Take me home with you!