It was a W.C. Fields joke, in response to mylife's photo of Fields in #33. After I posted it I realized that very few people would understand.
From here:
On movie sets, Fields kept handy a vacuum flask of mixed martinis, which he referred to as his "pineapple juice". One day a prankster switched the contents of the flask, filling it with actual pineapple juice. Upon discovering the prank, Fields was heard to yell, "Who put pineapple juice in my pineapple juice?!"
Well, I learned something. Thanks.