Definition
Having a hard, strong, and determined mindset.
How People Actually Use It
Used to describe someone or something that is totally hardcore intense. Not just used to refer to a person who is die-hard, but also used as an exclamatory "wow." Or to describe how much you enjoy something. Many tough as nails uses for tough as nails.
"Example 1 Look at Jack Bauer, jumping from rooftops kicking ass, tough as fucking NAILS. Example 2 Rachael Ray's birth chart is ALL Virgo. Every planet is in Virgo. Tough as nails Virgo. Example 3 Erica is a tough as nails jenny lewis fan. Rilo Kiley, Jenny Lewis all the time. Hardcore, tough as nails Jenny Lewis fan."
Source: Urban Dictionary