Unless you’re extremely unusual, your basic assertion is false. If you go to bite something, you very rarely merely push it upwards with your lower jaw – not least, that would only work with things under your control. There’s a definite, firm downwards movement of the upper teeth towards it as well. Make a quick, pretend “snapping” gesture – like you’re biting at a finger that someone is pointing at you just in front of your face, say – and you’ll almost certainly see that your upper jaw doesn’t simply remain still; rather,m it goes up and then down. And given that your eyes are fixed relative to your upper jaw, that means that, in the actual bite, your primary sensory organs are also moving downwards. So – whilst the origin of any phrase like this is pretty much guaranteed to be lost in the mists of time – it certainly makes a degree of (literal) sense to talk about a bite as “downwards”. Even though your lower jaw may end up doing the bulk of the actual work.
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