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1.
Probably came from an inexperienced photographer asking what to say.
Photographer: "What am I supposed to say?"
Person in picture: "Say cheese" (as in a cheesy joke)
Photographer: "Say 'cheese'!"
by Prometh September 13, 2006
 
2.
Phrase uttered before taking a picture so it looks like everyone is smiling even though someone would rather either frown, snarl, sneer, or make an otherworldy strange or unpleasant face.

A conspiracy to get unhappy or enraged people to smile in pictures.

A conspiracy to get more people to eat cheese and wine after taking pictures.
Photographer: Say cheese!
Lauren, Dave, Jessica, and Ben: Cheese! *smile*
Barry: Fuck you! *snarl* *flicks off camera*
*click**flash*
Photographer: Smile for us, Barry. We're not stopping until you smile for us!

Three odd hours later:
Photographer: Say cheese!
Lauren, Dave, Jessica, and Ben: Cheese! *smile*
Barry: Fuck you! *frown* *flicks off camera*
*click**flash*
Photographer: Smile for us, Barry. We're not stopping until you smile for us!

Brian: Say cheese!
Cristy, Ben, Mike: Cheese!
Ben: That's strange; I have a strange craving to scarf down some Gorgonzola with some Bordeaux!
Mike: Same here! I'm gonna gorge myself on Wensleydale 'n Chardonnay!
Cristy: Let's buy all the cheese and wine we can, guys!
by someone took my name =| April 19, 2006