This one's actually not that hard to explain. Shirts can be worn under your armor, and these are the only two kinds of shirts. Normally, a shirt adds nothing to your armor class, but it can be enchanted (up to +7 safely), so it's nice to have one if you can find one. The problem with the Hawaiian shirt is that shop keepers will assume you're a tourist, and charge you extra.
> Absolutely cannot tell if you're fucking with me, or if this is an actual game mechanic
Welcome to the world of nethack! The amount of special case situations that have been programmed into this game over the last few decades are truly amazing. (Mostly because it somehow still works.)
I still remember the first time I ran into a kitchen sink.
NetHacks DevTeam are so famous for their special cases that the TVTropes article for obscure special cases was named after them. Here's the subarticle for NetHack examples:
> Additionally, throwing a cockatrice corpse up (<) will result in it hitting you on the head and, unless you're wearing a helmet, petrifying you instantly. Cause of death? "Petrified By Elementary Physics"
Some of these I knew, most I did not. Thanks for sharing that.
Oh my, there's some really magnificent stuff, I started giggling while reading this:
The Quantum Mechanic monster is basically one big physics joke.
Its attack will randomly teleport you, and eating a Quantum
Mechanic corpse will give you the intrinsic speed ability (or
remove intrinsic speed from you if you were already fast). Upon
death, the Quantum Mechanic also has a small chance to drop a
box. When you open the box, a cat named Schrodingers Cat will be
found inside. 50% of the time, the cat will be alive, and 50% of
the time, the cat will be dead.
What brings this into The Dev Team Thinks of Everything territory
is that if you look at the game's source code, you discover that
unlike every other box in the game, the state of the box's
contents (whether the cat is alive or dead) is not determined
until you open the box and look.
Another fun one is to fall down the stairs while wielding a cockatrice corpse in your gloved hand. Or getting engulfed by a purple worm and subsequently regurgitated onto a cockatrice corpse laying on the ground, petrifying you.
