There are a number of definitions to this phrase, two of the commonest being:
1) Someone whose behaviour does not conform to society’s norms. It can be applied to a broad range of people, from those who are slightly eccentric to others who have completely lost touch with reality and need psychiatric help. The phrase can even be qualified to show the degree to which the person is affected. If the person was only mildly eccentric, they would be described as 'a bit of a whack job', if the behaviour was rather more weird then they might be described as '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).
1) If you’ve got problems with your car, see Georgie; he’s a bit of a whack job, but he can fix anything.
2) A bloke was found dead in the boat camber, looks like it was a whack job!
by AKACroatalin February 15, 2017
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There are a number of definitions to this phrase, two of the commonest being:

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).
1) Jackson Galaxy is a bit of a whack job, but he knows a hell of a lot about cats (he's not bad on guitar either).

2) Spanner was found dead in his flat, looks like it was a whack job!
by Croatalin January 09, 2014
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Jana was taking selfies at the top of "Jungle Jim". She is a certified whack job!
by Jimbo bob June 25, 2018
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a crazy person, especially one whose is in some way dangerous
I can't understand why they would let those whack-jobs have a say in things.
by The Retrun of Light Joker April 23, 2011
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1. A Person for whom failure is so consistant that they are slowly driven into madness.

2. Someone who partakes in unbelievably odd behavior that a reasonable human would avoid.

See LaToya Jackson
"Man.. every since billy lost his fast food job for the 12th time he's been acting like a total Wack-job"

"Prisoner #46729675 has been wiping his ass with his hands & not washing them for weeks on end.. What a Wack-job
by Platinum Pistolz June 15, 2004
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