A FUD is also a "female urination device": this is a thing that allows people who don't have a penis to urinate standing up, without having to pull down their pants and underwear. They are mostly marketed to hikers, backpackers, and rock climbers - in general, people who are likely to find themselves in a position where taking off layers of clothing to pee is embarrassing, inconvenient, or impossible.
In form, a FUD is a plastic funnel designed to fit between the labia over the urethral opening. It can take a bit of practice to get it to seal against the skin and not leak, so practicing at home where cleanup (and possibly a change of clothes) is easy is recommended before trying it on a trail. Also, a number of women (myself included) find that the hardest part of getting used to using one is overcoming several decades of training to always sit or squat before urinating.
Several companies make FUDs: the brands I know of off the top of my head are PStyle, SheWee, and Pibella. They vary somewhat in size and shape, so one or another will be easier and more comfortable depending on individual differences in anatomy. My hiking partner uses a PStyle and I have a Pibella - the Pibella is a much smaller profile (one reviewer describes it as being rather like peeing through a straw), but I find it's correspondingly easy to use around several winter layers.
I decided I should look into a FUD a couple years ago while hiking Sand Canyon in Colorado: being desert, there weren't any convenient thickets of trees to duck into, and I eventually had to resort to stepping off into a dry watercourse and hoping no one came in sight. It's proved its worth on an early-winter expedition to the Adirondacks a few days ago; it's much nicer than having to be half-naked in below-freezing weather.