"i don't know" or more specifically,
"I'm not sure I completely understand that tangential or advanced topic either, and I sure as hell don't want to risk making a fool of myself by being caught explaining it wrong."
It can also be used to mean "Even though I understand it, I'm supposed to be teaching you something else and we need to spend our time doing that." Like when someone asks a question about JavaScript in the beginning HTML course I teach. But the meanings that Maxwell gives are probably more common.
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