I've heard this expression several times in TV shows, but I couldn't find it anywhere. Is it like a small "piece" of some spread? Can I say for example:
"Shmare me some Nutella"?
Can I have a shmare of Nutella on my toast?
Just to add on to Lambie's answer: As with the Jewish holiday of "Hanukkah" (or "Chanukkah", "Chanukah" etc.) there is no single correct spelling for many of the Yiddish words that you might hear in regular conversation. This is because the words have been transliterated from a language that has sounds that don't appear in English.
The most common spelling I've seen is schmear, because the "sch" combination most closely resembles the Yiddish pronunciation, although most English speakers just say it like the typical "sh". So shmear or shmeer would not be incorrect.
There are dozens of similar Yiddish words that have made their way into common English -- kvetch, glitch, shlep, shmaltz, shmooze, schmuck, spiel (pronounced "shpeel") and many others. In some amusing cases the words have become so common that people use them without thinking. For example bupkes (or bupkis, or bubkis) recently appeared in the Disney children's movie Moana, even though the original, literal meaning of "goat excrement" is roughly equivalent to saying "shit".
There are a lot of words of Yiddish origin that have moved over into AmE speech in certain geographical areas and social groups.
Common ones include: schmear (schmeer or shmeer, see below), spiel (a story, as in invented story or tale), schlep (lug things around) and nosh (to snack). There are also some common insults like: putz and schmuck. Schmear (schmeer) is a verb. Please see below. Many are very funny.
A shmuck is an "unintentional jerk" whereas a schmendrik is a "deliberate" one. shmuck is very common today. Click on the Jewish Lexicon link below and search for it.
New York and its burroughs are where one hears a lot of this usage. One recent example was the current (orange-haired man) president (who is not at all Jewish but comes from New York City), who actually referred to a politician being "schlonged". I knew the word but had never heard it used as a verb. Many people unfamiliar with the term didn't even realize how funny it was. It means penis, and he meant it as "to be screwed" (to avoid using the four-letter word here).
EXAMPLE SENTENCES from the lexicon link below. "Let me schmear some sun tan lotion on myself." or" "I'll take a dozen plain bagels and one sesame bagel with a shmeer."
Please note: for a person from a non-Jewish origin such as myself, I take delight in these great words. There are found in the writings of such American Jewish writers as Isaac Bashevish Singer (Nobel Prize for Literature,1978) and Saul Bellow. Many of the terms are rather funny (haha).
Here's a video of the "language of humor": Yiddish and comedy