If you discuss something that is taboo, then you might be meshuga.
If a person acts in a way that senseless, he or she might be a meshuga.
Mendel: Come on Heschel, schlepp on over to the butcher!
Heschel: Mendel, are you meshuga? that place isnt kosher!
Ruva: It doesnt matter, neither of you two are straight up jew.