The term "weak atheism" was invented by a fellow named George H. Smith in the 70's. Before that, everyone knew what an atheist was, there was no such disagreement, no such debate.
A bit late to the game, but I thought this worth mentioning.
I may be wrong, but it seems you're implying that a distinction between 'mere
disbelief', which is by definition a
lack of belief (support for this below in the link), and an active denial of the existence of (god), is a relatively new concept? If that's the case, you are incorrect. For quite some time, this distinction has been clear in dictionaries, which presumably reflect the way the language was actually used (that is what they do after all - dictionaries are descriptive rather than prescriptive, no?).
Atheism : Definitions details - tree of reason
In particular, the historical definitions further down the page are of interest, as they show that this distinction (lack of vs. denial) has been drawn for at least a couple of centuries. Nothing newly invented here. If you meant only the specific term 'weak atheist', then of course this is recent. But the distinction was long in play and well understood by the time Smith slapped a new label on it.
One other thing before I indulge in coffee...
I'm not sure if I saw this on these forums or elsewhere, but I found it rather instructive to demonstrate how one can merely 'lack a belief', and that to imply otherwise is to set up a false dichotomy. That is, to insist that one have a positive belief one way or the other on any given proposition is a fallacy. A simple version;
I am in contact with someone on the internet for the first time. We have not met or spoken before and have no knowledge of each other. This person says 'I live in Detroit'. The implications of this false dichotomy are that I have only two choices;
1) I believe that this person does live in Detroit.
2) I believe that this person does not live in Detroit.
I think it obvious that these are not the only choices available. In this case, I simply lack a belief in either. There is no positive belief involved, nor could there be, rationally. It's a simple exercise to replace 'this person lives in Detroit' with 'a god exists', and the outcome is the same.
Oh, and for those that wish to use the rather tired 'are rocks atheists?' argument...don't be daft. If you can't understand the simple idea that any discussion of an application of belief, or lack of thereof, requires and entity with at least the potential of believing (anything at all), then a rational debate with you is...unlikely.