Words left unsaid

Talking about something that should have been said but wasn't, a number of paraphrases come to mind:

1 Words that remained to be said.
2 Words that remained to say.
3 Words (that were) left to be said.
4 Words left to say.
5 Words left unsaid.

And someone has come up with:
6 Words that stood to say

I don't really understand 6.

Which of the above sound natural to a native speaker?

Thanks