The holy anorak is an ankh symbol, wearing a small anorak.
The meaning of the holy anorak is that it's the 'ancient Egyptian' symbol of involuntary celibacy.
It was invented as part of a thread on Usenet by Terry Pratchett- the joke extended to the point that people were able to actually buy these items made of silver, pewter or gold; and some people with a sense of humor actually purchased them and placed them with pride (ok, humour) over their computers.