A derogatory reference to a male to female transsexual. An implication that the person is odd, in a disparaging manner.
Can also be used to refer to one who has to go and get off, to jerk off.
Also, could be used to refer that another person, may need to get off, such as in the phrase "he needs a wack (whack) job."
Mainly used, to denote someone who others consider very odd, mental perhaps, or simply deranged.