In the 1950's Dad never called them smoothies. He called them his health drinks.
I do remember him saying in the early 60's they go down smooth. Dad was always inventing new drinks. He refused to drink milk. He used protein powder, sometimes soy milk and for a time he bought goat's milk. He made his own version of Tiger Shake. He was experimenting. He wanted to find a way for me to gain weight. He was convinced I was anemic and needed to gain at least 5-10 pounds for my height. I was naturally thin and no matter how much I ate or drank I never gained weight. I liked the attention Dad was giving me so I drank and ate whatever he prepared.