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Used in older movies and cartoons to show the diner is happily anticipating his meal. Often followed by the disappointing reveal that the dinner is: very small, like a single pea; not food, like Chaplin's Tramp boiling a boot; or is still alive, as in many Looney Tunes cartoons. The last option will result in "dinner" running or flying away.
I think it is possibly a Forgotten Trope, unless newer cartoons still employ it.
One of these cartoon things I've never seen in real life (might be a Forgotten Trope) - before eating, the character will noisily rub both sides of his knife against his fork, possibly to sharpen it. The motion works both of a nonverbal demonstration of gluttony, and to clarify that the art, simplified as it is, represents sharp utensils.