

There are two ways to shake your head -- up and down (also called a nod), which is an affirmative gesture in many cultures, and from side to side, which is a negative gesture. This is by no means universal, but it's true for English speakers. Both Bruce and Yuuri are making the same head-swiveling gesture, the former to signal to the waitress not to serve him another drink; the latter to disagree with Conrad.
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HUBLOTt I wonder what "shake one's head no at someone" means.Shake one's head from side to side to signal 'No'.
HUBLOTSo, I have no idea why it is necessary to add "no" after "shake one's head"?It is a style choice. It also makes the meaning clearer.
deadratBecause you can shake your head (up and down) to mean "yes." In your second text,I don't agree. In BrE in such contexts., shaking your head means 'no', and nodding your head means 'yes'.