said to mean that more surprising or exciting things are likely to happen

If you thought that show was amazing, just you wait. You ain't seen nothing yet!

Let me tell you there's more where that came from. You ain't seen nothing yet!

That was just the start - you ain't seen nothing yet.

If you think these sales figures are promising, you ain't seen nothing yet.

I've only just begun - you ain't seen nothing yet.

See also: see verb (USE EYES)