There aren’t a lot of clothing options for people when they reach a certain weight. Shirts only go to Extra-Extra-Extra-Extra Large, and pants can only stretch out so much before they burst at the seams from the tension. The Muumuu is an article of clothing which helps address this distressing problem. Coming from the island of Hawaii, they are usually covered in floral prints, and possess more than enough room for even the most whale-like individual to slip inside. This is probably because most of them are just an enormous sack with arm holes cut into the side.
The unflattering name is just a coincidence. It actually means nightgown, and is not intended to refer to any possible sounds that the person would make in a field right before drunken college students snuck over and tipped them over. Still, the muumuu is not a particularly admired form of dress, and even the people who wear them acknowledge that it’s mostly out of necessity. There’s not much else that they can drape themselves in, so they might as well choose something that’s halfway comfortable. Any unintentional gender confusion caused by their voluminous nature is just an added perk.