Outy,
I don't think it's used a pejorative that often, these days, it's taken to be the 'correct' terminology, especially given the recurrent trouble people seem to have with the concept of gnosticism.
Maybe, though "you're a weak atheist" implies to me the meaning, "you're
just a weak atheist" as if that somehow diminishes the position.
I'm reasonably certain that the origins of it, though, come from the school of thinking that says atheists should have the courage of their convictions and prove their case.
But provided you stick to what "atheism" actually means - "without gods" - the position
is proved inasmuch as the arguments made for gods can be shown to be fallacious. It would be a fallacy too though to reach beyond that to attempt a, "and all arguments for gods there ever could be would be fallacious too" because that's an unknowable.
I don't know the origins, and I can't find anything on them, but I can't imagine any other reason for such emotive classifications. I've also seen them classified as 'positive' and 'negative' atheism (Anthony Flew?), which are less immediately emotive and slightly more justifiable, but is still open to implications, or 'hard' and 'soft' which is even worse.
I agree. I'm a "strong" atheist in that I'm strongly of the view that arguments
for gods are hopeless, but that's a different category of argument from a level 7 "definitely no gods" type atheist, for which there doesn't appear to be a word.