In Pathfinder, I seem to recall a specific type of undergarment beneath armor increases AC by one. It is not the armored kilt, and it does not increase armor penalty. From what I recall it's a mundane item that's some variant of silk. Anyone recall the type of garment and the source?
The armored kilt can be added atop other armor to increase the AC by 1; it also makes the armor one type heavier. See the Armor section of the d20PFSRD for more. It was first published in Adventurer's Armory.
Some people for some reason make a mistaken assumption that the haramaki can be stacked with other armor; it cannot. Same thing with the Silken Ceremonial Armor, which while it may be worn over other armor, says explicitly:
It's possible you are thinking about this previous question here about using ceremonial silk with a kilt for the +2 AC total: Does this armor set work RAW?
There's the aranea silk special material that gives some DR under armor, but that's third-party cheese.
Depending on how vague your recollection is, the Amulet of Natural Armor is the most usual thing that can actually stack with armor.
It might be the Snakeskin Tunic. It's a Chest slot magic item.
Armour and Chest slots are different in Pathfinder, so you could wear the shirt under (or over) a piece of armour, but unfortunately you couldn't benefit from both at the same time. At least in terms of the bonus to AC.
Armour, whether enchanted or not, grants an armour bonus to AC. This means that you would only benefit from the greater of the two bonuses (probably the armour, as the shirt only provides +1) as they do not stack. You would still gain the bonus to dexterity and saving throws against poison.