To overreacte, to throw a fit, to lose your temper; usually over something small
Also can be flapping your/a/my cracker, flapped your/a /my cracker
Bride: Oh my god! The white of the cake doesn't match the white of my dress! The wedding is totally ruined,we might as well just stop now!...
Groom: Honey, don't flap your cracker...
Guy 1: Where's Sue?
Guy 2: I don't know, she jusk flapped a cracker and took off.