An exclamatory phrase used to signify that matters have reached a point of no return. Nothing can be done. The situation has reached a low point or a level of finality. Alternatives to this phrase include "Case Locked" and "Case Welded," which indicate even higher levels of a situation being poor or hopeless.
Girl 1: I met a guy at the bar last night but but he's moving across the country next week and I'll never see him again.

Girl 2: Damn, case closed. You're locked.

Girl 3: Nah, this situation is case WELDED.
by FrenchOnion May 17, 2021
when you're telling someone the tea and instead of saying "period sis" you just say "case closed"
Example 1
girl 1: " that girl was trying to make me jealous with a pic of her and my ex"
girl 2: "really?"
girl 1: "yeah, and I already knew she was less than me but that proves it even more. case closed"

Example 2
"I hit her with the facts and that's that, case closed"
by Yeetis Man April 14, 2020