If you don't admit that you don't know something, then by your own doing, you diminish your chance to learn something new. You stop learning when you start saying you've learned all there is to learn. Wouldn't it be more sensible to simply come out and say, "yeah, I totally have no clue what the heck THAT'S all about. Care to tell me more?"
No wonder there are so many folks who finish others' sentences for them. (Yeah, because you know everything, including what everyone else is thinking. THAT'S why you can finish their thoughts for them.) There are far too many know-it-alls roaming the world now who probably don't know everything at all. I'm sure you and I have met our fair share.
The thing is, I believe you don't have to know everything. But goodness, I hope you DO know something. Just be willing to learn new things as well. Better yet, stop making fun of those who are willing to learn new things, just because you're not that way.