A mad or extremely eccentric person:he has established himself as a paranoid wack job...
The film does exactly what it sets out to do -- create genuine suspense about a teenager's fate in the hands of a predatory whack job -- and does so within a fresh conceptual framework.
We understood their frustration, but still thought they were total whack jobs.
You may not like him, but by no means was the man a "wack job."