I don't know that Jews get to take credit for this word, but, then, credit is never given, it is always taken. In Omaha we invented one of the great Jewish sandwiches, the Reuben, and, son of a gun, wouldn't you know it, along comes Reuben's Deli in New York to insist that, no, they made it. Like New York doesn't have enough, they need to steal a Midwestern sandwich.
Shmear means "smear" in Yiddish, but it means the same thing in German and related languages. I find references to a Pennsylvania Dutch food called schmear-
But no matter. If the Pennsylvania Dutch brought the word to the New World, the Jews ran away with it. Indeed, its secondary usage, which is to mean "bribe," has been associated with Jews since at least 1785, when Englishman Francis Grose included it in his "A Classical Dictionary of the Vulgar Tongue." This usage made it to America and never went out of style -- this past June two Florida politicians held competing bagel breakfasts, one of which was roughly ten times more expensive than the other. The competing campaign immediately put out a critical, if winking, press release "It isn’t clear what they can expect to gain from opting for the costlier bagels. Does that come with a schmear?"
Of course, the competing campaign could always claim they were asking an innocent question, as we do put schmear on our bagels. That's the way it's usually used nowadays, sometimes to mean cream cheese, sometimes to mean butter, sometimes to mean anything we could possible spread with a knife. There is a brand of almond filling from Love 'N Bake that insists it's a schmear. They also make cinnamon filling and call it schmear. They make a chocolate filling and call it schmear. One starts to get the sense that there is nothing Love 'N Bake won't put in a can and call schmear; I look forward to their haggis schmear.
So what is a schmear, really? Any condiment that can be schmeared, I suppose, so honey can be a schmear. Jelly is a schmear. Although, the less likely a Jew is to eat something in a deli, the stranger it will sound if you call it a schmear. Mustard is a schmear. Ketchup? No. Mayo? A schmear. Barbecue sauce? Get out.
Schmear should be used aggressively. Belly your way up to a deli counter and start hollering: "Does that come with schmear?" is a good starter. Or "What sort of schmear does that come with?" Or, "I'm on a diet, so go light on the schmear." And then tell them a little more, just a little more, it's just a diet, go ahead and pile it on. I think it's part of the reason schmear remains so popular. Nowadays, anybody can eat bagels, but it's still mostly Jews who make a fuss about schmear. It's a primal cry of ethnic pride. Try it. Go into contemporary bagel place and look over their menu, and then make a face at the person behind the counter and say "Canola butter? This is a schmear?"
Another usage of schmear is in the expression "the whole schmear," which, in essence, means "everything." You'd use it like this:
HAIM: Did you get your temperature took?
BERNIE: The nurse took my temperature, yes.
HAIM: And did she check your heart?
BERNIE: She was comprehensive.
HAIM: So she checked you all over?
BERNIE: Toe to tip.
HAIM: Even your ...
BERNIE: The whole schmear, Haim. The whole schmear.
Here are some examples of the use of the word "schmear":
The Cheaper the Crook, the Gaudier the Patter, Alan Axelrod: "Order a bagel and cream cheese in an oldschool New York deli, and you'll be asked if you want a 'slab or a schmear,' the cream cheese laid down in a slab or spread flat in a schmear."
Heidegger and Unconcealment, Mark A. Wrathall: "Consider the example of the salmon schmear. It is because I ordered a salmon, not a strawberry, schmear and because, in the context of bagel shops, one's order is generally fulfilled, that I am primed for my bagel to come with a salmon schmear. The pinkish color of the schmear in that context leads me to anticipate the fishy flavor of a salmon schmear."
Blood Atonement: A Dahlgren Wallace Mystery, Jim Tenuto: "'My claim to fame,' he boasts, 'is introducing a decent bagel and a schmear to the state of Montana. That, and klezmer music. You think any of these cowboys ever heard klezmer music before The Cowboy Vey Deli opened?'"