Rei made up this word and Megaduck when she didn't feel like using the word Superduck. There has been small debate over which of the three words has the most vulgar intentions. However, since the prefix giga- means one billion and mega- means one million, it would appear that Gigaduck or Superduck is the worst, since Super can't be scaled in numbers.
Rei - No I'm pretty sure that super is the least bad since it was part of the filter. I personally think that giga is more than mega, though, of course. >: ( I also clearly remember that we took into account Superman, Megaman, and Gigaman. Of the three, Superman is the toughest. :' (