It was ever thus informal
A humorous way of of saying things are always this way or something has always been this way
Twas ever thus, on the first page of our life's story, everything seems so bright.
Everyone gets married and has babies when they grow up. It was ever thus.
To not react or deal with something that is wrong or unpleasant
Unchanged, be stable.
If someone has a cast iron stomach, they have a strong stomach that can digest unusual food and even bad food without being ill.
Jimmy must have a cast-iron stomach, when he drank milk with lime juice and felt perfectly fine afterward.