The first instance of the use of this phrase was in response to criticism of The God Delusion by Richard Dawkins; a standard criticism was that Dawkins had not studied theology and was therefore unqualified to discuss evidence for or against the existence of God. This is fallacious because, although as a non-theologian he is not technically qualified to discuss the nature of God, as a scientist he is extremely qualified to discuss the nature of evidence. Moreover, any discussion on the nature of God (as anything other than a hypothetical entity) depends entirely upon first proving the existence of God as a real and tangible object.
Dawkins himself once said, when referring to the fact that he is not a theologian:
"Most of us happily disavow fairies, astrology, and the Flying Spaghetti Monster without first immersing ourselves in books of Pastafarian theology etc.Credit is given to PZ Myers for this logical fallacy. Click here for a recent post by PZ discussing the controversy within the skeptical community on this subject.