You appear to be using the concept that if A is B, then B must be A.
Although a spider may be an insect, this doesn't mean that an insect is always a spider.
Having started a new thread on this topic, your earlier comments aren't visible to your new readers.
In your new post, you compare shaking heads to nodding heads, but don't appear to provide any confirmation of your statement that wagging = shaking (as in saying "No").
In your previous post, amongst many unreferenced quotes whose context we are unaware, (you really need to quote your sources), you include:
Locals 'wag' their head in a wonderfully subtle way when they agree, BUT....they have a similar 'wag' when they are not satisfied, so careful attention must ...
Wag expresses specifically the motion of the head and body used in buffoonery, mirth, derision, sport, and mockery.
It isn't clear, as you didn't quote the context, but I would suspect the "locals" quoted may have been from the Indian subcontinent. Your text denies your current hypothesis, as it suggests that wagging of the head could represent yes or no, i.e. the equivalent of nodding or shaking the head.
Although not currently "politically correct", I can understand your comment about wagging the head representing making fun of someone, as in imitating someone in the paragraph above, where wagging the head (and possibly saying "atcha, atcha") could mean either yes or no (or even, I haven't understood a word you are saying, but I'm trying to be polite)!
In this world of trying to explain the nuances of the English language, context is absolutely vital.
Student or Learner