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.
Look at Jack Bauer, jumping from rooftops kicking ass, tough as fucking NAILS.
Rachael Ray's birth chart is ALL Virgo. Every planet is in Virgo. Tough as nails Virgo.
Erica is a tough as nails jenny lewis fan. Rilo Kiley, Jenny Lewis all the time. Hardcore, tough as nails Jenny Lewis fan.