I'm not a grammar expert and English is not my first language but I think I used to say this before and I just ended up taking out the "what it is" and changed it for the thing I'm trying to remember:
There's a thing that I don't know the name of
Or
There's a thing that I don't know how to describe
Or
There's a thing whose purpose is a mystery to me
Is that what you're refering to? Sorry if it's not. I don't think any of the first three examples are correct, or at least they sound really weird to me.
Please do correct me if there's an English mayor somewhere though!