|^ rust trap|
On the player
Any worn or wielded item can potentially become wet. For unprotected iron items, this will cause rusting. Whenever the trap is activated, it randomly picks an armor slot to target, instead of looping over every armor slot. This means no item is strictly 'guaranteed' to become wet.
If you are polymorphed into an iron golem, you will take damage equal to your monster form's maximum HP, killing your monster form and reverting you to your natural form. The half physical damage extrinsic applies. 
Items worn or wielded are affected, similarly to those of the player.
Dig a pit into the ground while standing on the trap, then fill the pit.
In the early game, you should wear a cloak to protect your body armor.
You can use a rust trap as an infinite, if somewhat slow, source of blanking scrolls and spellbooks and diluting potions, by wielding them, removing any other rustable gear, and repeatedly triggering the trap.
Rust traps were added in NetHack 1.3d
- A gush of water hits you on the <body part>!
- Your armor in the corresponding slot takes water damage.
- A gush of water hits you!
- The trap hit your body armor slot, or you are an iron golem.
- You are covered with rust!
- You are an iron golem and were hit by the trap. Your monster form will be killed unless you have half physical damage.
- A gush of water hits <monster> on the <body part>!
- A monster's armor in the corresponding slot takes water damage.
- <monster> falls to pieces!
- An iron golem was hit by the trap and died.
- May <monster> rust in peace.
- As above, but the iron golem was tame and was out of sight.