What does the sentence mean and in what kinds of situations is it used? Some postings say it means "you can find my sympathy between shit and syphilis." But so what?
What it means is "I am unwilling to give you any sympathy."
The point is that the only place you will find sympathy from me is in a list of words, a dictionary, since I have none in my head or heart.
This is a pretty ugly way to speak, and is further emphasized by the second phrase, using the ugliest bracketing words they can think of (shit is before sympathy in the dictionary, and syphilis is after it.)
Moreover, it doesn't even really work as a dismissal, since the "sympathy" in the dictionary doesn't actually "belong" to the speaker.
Ugly and incorrect isn't a combination that reflects well on the speaker.
It's an expression not only of total lack of sympathy, but contempt - either for the person you're talking to and their problems, or for the concept of sympathy itself. Obviously this is extremely rude, and you would only either say it to an enemy or to a very good friend who knows you're joking.
A more common variation: "Where can you find sympathy these days? In the dictionary, between shit and syphilis" - is a cynical, off-color complaint about the human condition. In this variant, the speaker is essentially saying that s/he has experienced personal tragedy, but can't get anyone to sympathize. The speaker is complaining, but half-jokingly; s/he doesn't expect to be taken seriously.
My father used this saying when I was growing up. He was born in 1921 and served in WW2 in the Pacific. I was shocked while watching "World at War" when an excerpt of an interview with a (disabled) service man came on. He asked an Orthopedic Surgeon from Baltimore about "sympathy for the disability" and the surgeon answered, "it's in the dictionary between shit and syphilis." Now I'm trying to find out where that saying originated. I never heard it from anyone other than my dad.
If you think someone is expecting sympathy from you, and you want to be as rude as possible in denying it to them, then you'd say something like:
"If you're looking for sympathy, the only place you're going to find it is in the dictionary, between shit and syphilis."
"If you're looking for sympathy, go look up a dictionary between shit and syphilis, that's the only place you're gonna find it."