I’m not a grammar expert, but I would say “there’s a thing and I don’t know what it is” or “there’s a thing but I don’t know what it is.”

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!

Simpler: I don’t know what this/that thing is.

Basically trying to say: there’s this thing that I can’t remember the word for/don’t know exactly, but I know it exists and need it for context.

It is awkward, but many dialects compress, forgo, and bastardize sentence structure depending on where you’re at.

Describe the situation in context. None of those phrases sound natural.

The closest phrase I can think of is: “There is a thing of which I do not know.”

Awkward af phrasing, though, as others have stated.

I don’t know what this/that thing is.

I don’t know what that thing is.

I don’t know what that thing is for.

I don’t know what that is.

Any of these work for what you are trying to say?

What dis? Da fuq?

I don’t think I’ve come across that before, but I’d say it depends on what is meant:

• I don’t know what that thing is.
• There is a thing, but I don’t know what it is.
• There is a thing such that I don’t know what it is. I.e., I do not know what all things are.

There may well be some other ones, but I don’t know what they might be.

There’s a thing I can’t identify.

There’s a thing I don’t know about.

There’s an unfamiliar thing.

All the formulations you wrote indeed sound either ungrammatical or unwieldy to me.

There’s a thing of which I know not what it is

If you want to be a bit poetic: There’s a thing I know nothing of.

If you don’t know, I sure as hell don’t.

It sounds OK if you say it in French

“There’s an unfamiliar object”