2. Someone who partakes in unbelievably odd behavior that a reasonable human would avoid.
See LaToya Jackson
"Prisoner #46729675 has been wiping his ass with his hands & not washing them for weeks on end.. What a Wack-job
1) A person who exhibits abnormal behaviour, this can vary from mild eccentricity to full-blown, froth at the mouth insanity. The degree to which the person is a whack job is usually given by qualifiers to the phrase. Mild eccentricity would be indicated by saying the person was 'a bit of a whack job' stronger symptoms of eccentricity and the person would be 'a proper whack job'. If the person was seriously insane then they would be 'a total, out-and-out, raving whack job'.
2) A professional killing, also known as a hit. The whack part comes from the victim being killed (whacked), the job from the fact that this is done for payment by a professional (it's his or her job).
2) Spanner was found dead in his flat, looks like it was a whack job!