The dismissive nod of approval given to waiting staff in restaurants who come over to ask how the food is while you are in the middle of eating and have a mouthfull of food, and therefore can't answer them without either spitting food everywhere or making them wait. Assuming you were brought up properly and not dragged up by homeless gypsies.
Waitress: Is the food ok?
Customer: <chewing, nod>
25 seconds later
Customer to Guest: Was my fuckoff nod obvious?
Guest: Yes. You shouldn't have flipped the bird at the same time.