I would assume everyone here is referring to d2data.net's info. Look here to see where people are getting the idea.
This mysterious absorb property can be found in the mpq archives. To the person who said they couldn't find it, you didn't look though all of them. The column being referenced to is not far along in the armor.txt file located in patch_d2.mpq. It is called 'absorbs'
AFAIK the absorb property does not actually work ingame. I believe it was supposed to act as a damage reduced by x modifier, but it never made it to the game code. Yes, the property is defined in an mpq archive, but that does not mean it exists ingame.