Visual Shorthand, cue, cliché, whatever - you can Google-image licking lips food and find many images from cartoons, ads, clip-art etc. of people licking their lips in anticipation of eating. It’s common enough in old school advertising, and persists in today’s clip-art (example).
But in reality, I nor anyone I’ve know (admittedly somewhat less than 7 billion people) have done so except in a comic manner. Nervous tics, to wet dry lips, to clear smears of pasta sauce on their lips, yes I’ve seen people licking their lips, but just looking at food they haven’t tasted yet? Nope, and on the face of it doesn’t seem to make a lot of sense.
So where did this common image come from? Different cultures? Pets? Out of the blue in a long distant past? Where?
Sadly for the purposes of this thread, searching for licking lips on this board brings up many posts related to licking and a lady’s other set of lips…