But it's misplaced and uses the wrong relativizer, if
I've correctly understood your intended meaning.  "As
a word that didn't know each other" is non-sense.  So
I re-parsed it as "people who didn't
(frequently/ordinarily) communicate with each other".
If I've erred in re-parsing then I don't understand
you either.

Hmm...  Let's see:

>  I've heard
> people use it as a word that didn't communicate with each other
> (so they couldn't have gotten it from one another).

I don't know about the wrong relativizer.  You can just move it
next to people and it's standard:

I've heard people that didn't communicate with each other use it
as a word.

You can use "that" there; there's nothing wrong with it.  As for
MISplaced, well, that's a matter of opinion, isn't it?

"A male love inevivi i'ala'i oku i ue pokulu'ume o heki a."
"No eternal reward will forgive us now for wasting the dawn."

-Jim Morrison