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

Part of me feels like this is a question surgically designed to make someone go insane.

It sounds OK if you say it in French

“There’s a thing I’m unfamiliar with”

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?

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

What dis? Da fuq?

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.

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!

“There’s a thing which I can’t identify”

or

“I don’t know what this is”

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.

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.

I’m struggling to think of a context where you’d say this where you couldn’t just say “I don’t know (about) that thing” or “there’s a thing I don’t know”.

That there is a thing is kind of implied.

Best sounding recommendation probably depends on context and ‘the thing’:

There’s a concept I don’t understand.

There is something in the box I don’t recognize.

There is a feature of the coffee machine I haven’t figured out yet.

There’s a Greek word in the original text that I don’t know.

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.”