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'.
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