What is She Hiding?

As I sit here in my khaki colored pajama pants and ribbed yellow tank top in front of my air conditioner, I've realized I've become my mother. She turns on the a.c. the moment it hit 70 degrees (Fahrenheit). But then I came across this image below, and realized how incredibly comfortable I am. I really can't believe that women used to wear this kind of thing, I mean really, who decided that this would be the new trend?
From the MET Costume Institute which is still on my to-do list