Armpit Stains

When I was younger, and my family was less well off financially, we would often wash cars at home. We used old clothing as washing and drying rags. Although most of my memory from that time is nonexistent, one thing that I do remember are the rags which were my father's old undershirts.
The pit stains were horrendous, and I made sure to avoid using them if I could.
These stains were so bad that the cloth in that area was hardened. And this isn't like old, air-dried, stiff cloth. Its feel was more reminiscent of the hard plastic you would find used in toys. However, they also did not seem to yield at all to pressure, not much less crack.

I still have no clue how something like this can happen from perspiration, and frankly, my disgust outwieghs my curiosity. Also, how my dad could let it get to that point as well remains a mystery better left unsolved.

Jam it back in, in the dark.