I'm still confused about the meaning of こと. But I think it means "this," "that," "about," and maybe "it."
So, this sentence could be translated as: "I've never heard of this/that/it!"
But I'm not very sure ...
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The structure Past verb + こと + ある/ない means that you Ever/Never Performed a given "action".
日本に行ったことない。Nihon ni itta koto nai. - I have never been to Japan.
Now a conversation about two people talking about Nattou (Japanese fermented beans)
Person1 - 納豆を食べたことある？Nattou wo tabeta kot aru? - Have you ever eaten Nattou?
Person 2 - はい、納豆を食べたことあるよ！Hai, Nattou wo tabeta koto aru yo! - Yes, I have already eaten Nattou!
So, 聞いたことない probably means "I have never heard about that."!
If you want to be a bit literal "The thing of having heard that does not exist"