I think it refers to an inner strength or inner courage. It is an old word that really isn't used anymore.
Here is a definition that I found:
a person's ability to cope well with difficulties or to face a demanding situation in a spirited and resilient way.
that’s mettle, this is; n n e t t l e
I’ve heard of and seen nettles before. I was just asking more about the site before I clicked it, rather than a definition. i hadn’t noticed that site before so I was curious if it was legit