If someone is as tough as old boots they have a very strong character and can cope with bad conditions and a lot of work.
If something is as tough as old boots it is very strong and does not break or tear easily.
Examples of use:
1. My grandmother is as tough as old boots. She lives alone on her farm and works outside all winter.
2. I rode this motor bike across Europe. It's as tough as old boots.
Image by Vince Alongi