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A whack job nutcase who goes on a murderous rampage for any number of reasons, including but not limited to mental instability, political/ideological disputes, delusional government conspiracy theories, or personal socio-economic woes.

As defined by Pima County, AZ Sheriff Clarence Dupnik on msnbc.
That fuckin' whack-job Jared Loughner who shot AZ Congresswoman Gabby Giffords and 18 others is a prime example of a particularly troubled personality.
by dookeyboy January 11, 2011