Nickname
Sonny, the nickname comes from the grip required to depress the grip safety. Like squeezing a lemon to make lemonade. Actually, it isn't nearly as hard as squeezing a lemon, but it's 'cute' and therefore stuck.
The original 'lemon squeezer' was the S&W "Safety Hammerless" revolver from 1899 (yes, '1899') or so. I found a picture and brief write up on the old revolver on line by searching for 'Safety Hammerless'.
The second in the 'lemon squeezer' branch of the family was the Centennial (model 40 and 42 [airweight]) made from the middle 1950s or so until the great renumbering of the early '80s, to the best of my knowledge. Someone with greater access to the details may correct this if I'm wrong.
Why did Smith & Wesson decide a revolver needed a grip safety? I have no idea. It must have seemed like a good idea at the time.