salandrick I am going to put it plainly. Every time a "sign" was chosen it quickly became not a secret and people have been harassed because of it. Anyone remember the old apple with a bite out of it pin (take a bite out of life but it also was known as the original sin)? If you do you know it became the accepted thing but you have to ask where is it now and why is it no longer a thing. The pineapple comes up so much because everyone knows the "meaning" and how much of a joke it is. No one that is serious (or not many) use it as some kind of "secret sign" because it is not a secret. It is the longest running "joke" simply because it has long been a sign of hospitality.
In the 1700's the standard was if the pineapple is out it is okay to visit because the husband is home. Back then it was considered improper for a man even with his wife to visit the home of another man if he were not there but his wife was. The pineapple was a symbol to show it's okay to visit. The upside down pineapple started being a "thing" because it was generally assumed that women wanting company would display the pineapple but put it upside down. This may or may not have been true or since the pineapple was put up and down so much the nail was loose and fell out.
