doorknob's explanation is a direct quote from Wikipedia which cites one of the origins of the term as emanating from Robert Flint in his "Croall Lecture" from the nineteenth century.
It is identical to the "strong" -vs- "weak" flavors of agnosticism that I already described. It actually predates my explanation I gleaned from
this page on About.com. I believe the term
"agnostic atheist" so forth are just terribly confusing personally.
TCD John