Where does the word “Hubbub” come from and What does Hubbub mean?

The best we can do is to say “probably.”

That is, the term “Hubbub” for a confused din, an uproar of sound from a multitude of voices, was probably taken from an ancient Irish war cry.

The cry itself appears to have been abu! abu!, repeated over and over from the throats of yelling hordes.

Edmund Spenser, in The Present State of Ireland (1596), describes it:

“They come running with a terrible yell and hubbabowe, as yf heaven and earth would have gone together, which is the very image of the Irish hubbabowe, which theyr kerne (foot soldiers) use at theyr first encounter.”