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?
Quilted Cloth from Advanced Players Guide pg 179. This enhanced form of padded armor has internal layers specifically designed to trap arrows, bolts, darts, shuriken, thrown daggers, and other small ranged piercing weapons. When these kinds of weapons strike you, they tend to become snagged in these layers and fail to harm you. You gain DR 3/— against attacks of this kind. The special layers of the armor have no effect on other kinds of weapons. Effectively DR 3/ Vs Ranged (Bludgeoning or Slashing), or Melee (Bludgeoning, Piercing or Slashing) or Siege projectiles (& Giant's throw rock ability). This can be worn beneath other armor (except for padded)
Other AC Items:
The Neck Guard is made from hardened leather reinforced with a band of metal, this collar protects the wearer against vampire bites when worn around the throat. It provides a +1 armor bonus to AC against vampire bites or similar attacks that specifically target the wearer’s throat. Unlike most armor bonuses, the neck guard’s +1 bonus stacks with the armor bonus of light or medium armor, but it provides no additional bonus when worn with heavy armor.
The armored kilt is made of a thick cloth skirt with bars of steel hanging down from the waist and a ring of horizontal steel plates just above the hem. An armored kilt can be worn separately as light armor, or it can be added to other suits of light or medium armor. Adding an armored kilt increases a suit of armor’s armor bonus by +1, but it adds 15 pounds to the armor, lowers the maximum Dex bonus by 1, and increases the armor’s weight category (from light to medium and from medium to heavy). Adding an armored kilt to heavy armor does not provide an armor bonus increase.
This doesn't exist in first party Pathfinder options
Things that are kinda similar include:
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:
Used for ceremonial displays or occasionally worn (albeit with no additional benefit) over heavier armor, these robes consist of several layers of cloth and an outer layer of silk intricately woven with gold brocade designs and covered with metal studs.
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.
A snakeskin tunic is a tight, form-fitting shirt crafted from the scales of a giant snake.
When worn, it grants a +1 armor bonus to AC, a +2 enhancement bonus to Dexterity, and a +2 resistance bonus on saving throws against poison.
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.
Each type of armor grants an armor bonus to Armor Class. The armor bonus from a suit of armor doesn't stack with other effects or items that grant an armor bonus. Source
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.