What is the definition of Undefined Fumes?

"Undefined fumes" is a term that's like a secret ingredient in a recipe you've never tasted but knows you're trying to add to your life. It's a fancy way of saying there's nothing concrete or measurable about it - it could be anything from the scent of a freshly opened closet to the after-effects of a particularly heated debate.

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Now, here's an unsettling example sentence: "After the lab leak, the 'undefined fumes' from the synthetic virus seemed to whisper secrets in my ear, promising eternal twilight."