I believe the tern pre-dates the safety bar hammerless revolvers. There was an odd pistol that you had to squeeze in order for it to fire that was called the squeezer by the manufacturer in advertising. It was the Protector Palm Pistol is a small .32 rimfire revolver designed to be concealed in the palm of the hand. The entire handgun was squeezed in order to fire a round. I have found the term used in a couple of old gun books in reference to this type of pistol, but have not been able to find the references. It was called the Palm Squeezer pistol in the late 1800s and was round like a lemon, so no big leap to the lemon-squeezer term.
I believe that S&W was the first company to successfully introduce a quality safety bar hammerless revolver and soon after there were many copies on the market by firms like Iver Johnson, Harrington Richardson, Forehand Arms, etc. All were considered inferior to the S&W Safety Hammerless guns. Reading about these guns from publications back in the day, the term lemon-squeezer seemingly became synonymous with those inferior quality copies and that is why many collectors do not like the term as it applies to Smith & Wesson.