I'm pretty sure I read every word of the original Moria docs and could never figure this out.
You know how in Moria/Angband the Armor is written like..
Metal Brigadine Armor (-3) [19,+0]
Okay the 19 is the number added to your AC. The base protection so to speak.
The +0 is it's magic value.
What the world is the (-3)?
At first I thought it was a good thing, like a damage reduction per strike. But after seeing the Rusty Chainmail (-3) I started thinking it was more of an encumberence thing. However it doesn't seem to lower DEX, MOV, or anything else I've noticed on the Character Sheet. 100 tripe rations to the first person to solve this decades old question!