This article is from the Anime FAQ, by Steve Pearl with numerous contributions by others.
This is a question that has plagued anime fandom for some time. Some people have speculated that it could be a phonetic reading of an acronym (Much the way Mospeada actually stands for Military Operation Soldier Protection Emergency Aviation)
like Boomer = B_io- or BUMA =Big M_echanical Ugly R_eplicant Metal Android
But as always, Robert Woodhead has the definitive answer: "It does not stand for anything. The creators liked the feel of the English word (ie: one who makes a lot of noise, a lot of fuss) and one thing lead to another..."