How do you say something like that?
"There's a thing for which I don't know what it is" "There's a thing where I don't know what it is" "There's a thing that I don't know what is"
or (the one which I hear people say a lot but sounds awkward:) "There's a thing that/which I don't know what it is"?
To be honest they all sound awkward to me to varying degrees
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.