Stone Body (3.5e Martial Artist Maneuver)
From D&D Wiki
Sticks and stones cannot hope to break my bones, but I'll break yours soon enough.
Level: Hard 1
Prerequisite: Constitution 13
Description: When assuming this stance, you gain an effective +2 natural armor bonus to AC, and a +1 competence bonus to damage with any weapon with which you're proficient.
Special: This stance cannot be activated if you have moved more than 5 feet already during your current turn, and is immediately dropped when you move more than 5 feet during any subsequent round.
The natural armor gained from this maneuver is not actual natural armor, but rather the effect of hardening and tightening all the muscles in the body. It is treated as temporary natural armor and does not let you qualify for feats with natural armor as prerequisite.