(And I'm not talking about Don's pot...)
I was rather taken by surprise by a line in an early 1937 show where Jack and the gang are over at Phil's house where his mother and sister are staying. Jack asks about the wonderful smell coming from the kitchen, and Mrs. Harris says, "My POT roast." Big laugh.
Chalk it up to my own naivete if you will, but I had to have Hal Stone explain to me that it was really a reference to marijuana.