My infant great niece was sucking not her thumb, but both her index finger and middle finger at the same time. My mother used to call that "salt and pepper." had anyone else heard or used this term? Do you have any other special term for this action?
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Whether it was a thumb, finger(s), or even toes (all of which were seen in my 4 younger siblings) my mother just called it "sucking." And this was in the Midwest. The only other use for "salt and pepper" I've ever heard, other than their normal sense as seasonings, is to describe the appearance of hair, usually a male's hair, that is just starting to gray. But then, I'm sure you already knew that.
We have 'salt and pepper' snakes (speckled king snakes) in southwest Missouri.
Emmett