To laugh hysterically and uncontrollably. Sometimes considered perjorative, if used to describe someone laughing at something that wasn't that funny, or who has been laughing so long that it's now seriously uncool. Used heavily in the Maritime provinces of Canada.
Alex: "Yeah, man, it's gonna be great. I am gonna break weak!!"
Joe: "Did you see Mr. Johnson after Jenna told that joke in History?"
Alex: "Yeah man, he was breakin' weak!!"