What is a good idiom for deceptive smile?

Is there a good idiom that implies fake happiness in the same way 'crocodile tears' imply fake sadness?

Solution 1:

Not an idiom but you can say a sinister smile.

Edit: maybe too simple but: a fake smile, or an empty smile

Edit 2: I just found this online at Yahoo! Answers; I quite like this one:

Pan-Am smile > It is named after the airline Pan American World Airways, whose flight attendants would always flash every jet-setter the same, perfunctory smile.

Other alternative terms for a "fake smile" are: Botox smile, a say-cheese smile!

A fake smile only involves the zygomatic major muscles which are forced by a person in order to give a polite smile. The smile is then limited only to the lips and there is no contraction in the eye area.

The study of smiles is a part of gelotology, psychology, and linguistics.

And it may be good to know that scientific word for a "real smile" is "Duchenne Smile" which is named after physician Guillaume Duchenne who first recognized the muscles involved with this smile.

Solution 2:

It depends what you're trying to convey. If it's a lack of emotion, or a lack of interest, it could be "a pale smile", "a polite smile", "a perfunctory smile", "a mechanical smile".

To suggest that it's regularly used, "a practised smile", "a receptionist's smile", "a salesman's smile", "a politician's smile".

If there is any negative intent, it could be "an intimidating smile", "an icy smile", "a debt-collector's smile"...