I only know that about 4yrs ago due to the forum software that was in place at that time. You were required to type at least four charaters to post something, so I believe there is more to how the word "bump" came about.
I distinctly remember seeing the error kick back of you just went and type something like an "X" or a "C" to move the thread to the top.
The term itself may have been around for more than four years. I am only stating what I have witnessed over the past four years.
So I guess I will "bump" this as I have nothing more useful to say. Just being funny.