Nails for breakfast without any milk

It means your vary tough
Nails for breakfast with no milk was something from hat show spongebob squarepants wher the main character spongebob tries to go to a place called the salty spittoon

Fish: Welcome to the salty spitoon how tough are you
Other fish: how tough am i?! How tough am i?! I had a bowl of nails for breakfast this morning!
Fish: yeah so?
Other fish: Without any milk.
Fish: right this way sorry to keep you waiting

Nails for breakfast without any milk