To flex means to boast or show off about something. The phrase 'weird flex but ok' is said when someone proudly boasts or brags about something that most people would find either awkward, irrelevant or just plain weird. It's usually stated in a dismissive fashion.
Girl 1: "I'm dating my firend's dad, if we ever get married I'll become her stepmom!"
Girl 2: ... Weird flex but ok.
Person 1: "Yo, I finally hooked up with my cousin on Thanksgiving."
Person 2: ... Weird flex but ok.
originally meant to be used in response to someone who mentions something odd that they do or have done, the phrase has quickly become overused and obnoxious.
Correct usage:
DeShawn: I like to let a girl know when I've masturbated to her.
Jerome: Weird flex but Ok.
Common usage:
NFL Instagram: Bradshaw McPeterson is out for the season with a torn ACL
Idiots in the comment section: Weird flex but Ok
Jake: My wife went to the doctor and we found out she's having a child!
Dustin: Weird flex but Ok