Use it when you completely lack knowledge on a subject, not just when you're unsure.

Lack of Knowledge

It's an admission that you can't provide information or insight about a subject.

No Insight Needed

Use alongside phrases like 'at all' or 'whatsoever' to emphasize complete ignorance.

Reflects your own confusion, not someone else's misunderstanding of your knowledge.

Often a spontaneous reaction to a situation or question.

You might have some information but still feel puzzled enough to say you don't have a clue.