Shm-reduplication

Shm-reduplication is a form of reduplication in which the original word or its first syllable (the base) is repeated with the copy (the reduplicant) beginning with shm-, IPA. The construction is generally used to indicate irony, derision or scepticism with respect to comments about the discussed object:
 * He's just a baby!
 * Baby-shmaby. He's already 5 years old!

A classic example of shm-reduplication is "Latin schmatin", a slang phrase for binomial nomenclature.

Shm-reduplication has been used for humorous and ironic effect, as in the Radio Free Vestibule joke:
 * What would you like to have for dinner? Pizza?'''
 * Pizza-schmizza!
 * How about chicken?
 * Chicken-schmicken!
 * OK, what about Smorgasbord?
 * Smorgasbord-sch...   OK, sounds good

Phonological properties
Further phonological details revealed by Bert Vaux and Andrew Nevins' online survey of shm-reduplication can be found here.
 * Words beginning with a single consonant typically replace that consonant with shm- (table shmable).
 * Words beginning with a consonant cluster are more variable: some speakers replace only the first consonant if possible (breakfast shmreakfast), others replace the entire cluster (breakfast shmeakfast).
 * Vowel-initial words append the shm- directly to the beginning of the reduplicant (apple shmapple).
 * Some speakers target the stressed syllable rather than the first syllable (incredible inshmedible); a subset of these do not copy base material preceding the stressed syllable (incredible shmedible; cf. Spitzer 1952).
 * Shm-reduplication is generally avoided or altered with words that already begin with shm-; for instance, schmuck does not yield the expected *schmuck schmuck, but rather total avoidance or mutation of the shm- (giving forms like schmuck shluck, schmuck fluck, and so on).
 * Many speakers use sm- instead of shm- with words that contain a sh (Ashmont Smashmont, not *shmashmont).

Origins and sociolinguistic distribution
The construction appears to have originated in Yiddish and was subsequently transferred to English, especially urban northeastern American English, by Yiddish speaking Jews. It is now known and used by many non-Jewish English speakers.