While this ain’t a common term in Australia and is not listed in the Macquarie Dictionary, it’s more used as a colloquial term in the US where it means “an ignorant or unsophisticated person”, like a hillbilly perhaps. There could be a few reasons for this. One is that such a person is so silly they would do something like knock apples out of trees with a stick just for fun, which is a wasteful pastime for many reasons. But also the apple is a symbol of education and if you’re an apple-knocker you might eschew education.
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