In this world human-like people have skin that is unbelievably difficult to cut through and I was wondering what biologically creatable material is flexible enough to act as skin but it also essentially impossible to penetrate with a medieval era spear.
Same size as humans with a 30% threshold if you need it.
Same size planet as earth with similar atmosphere and minerals, again minor changes necessary are ok e.g. 10% more aluminium or something.
It can be any material that could theoretically be made by a body e.g. some theoretical polymer.
Must be based upon a real material that is either believed to exist by reputable sources or does exist. Minumum handwavium, please.