1)The belief that having "shm" as a prefix of a word makes it Jewish/Hebrew slang
2)The belief that adding "shm" in front of a word presents it as having no value to the speaker. Usually used directly after the original word.
-Not tonight honey, I'm too tired.
-Tired shmired! I had a long day too, you don't hear me shmalowzing about it. Night after night it's the same old shmuck with you.
-Woah, "shm" complex much?