Japanese - what does "tennnen" usually mean?

As usual, I found this word through Japanese TV shows - at first it was subbed as "airhead" (i.e. ignorant), but now I've seen it to mean "a natural at ~" (i.e. brilliant at). Is one of these wrong, or does it depend on context?
Update: The context of the "a natural in ~" made it clear that it was that, and not any other meaning of "natural". The exact phrase was simply "てんねんだな" with the subs reading "A complete natural." Could the "natural" from "てんねんだな" be more like "[his... show more The context of the "a natural in ~" made it clear that it was that, and not any other meaning of "natural". The exact phrase was simply "てんねんだな" with the subs reading "A complete natural."

Could the "natural" from "てんねんだな" be more like "[his amazingness is] natural [as opposed to learned]"?
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