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!