Was bedeutet '' nun rate mal , ... '' auf English ?

I would say it literally means "have a guess" but its meaning is more like "you have no idea...".

In my point of view, "nun rate mal" is used as a rethorical instrument to pose a question and raise your opponent's interest in the conversation. For example:

Nun rate mal, wenn ich eben gerade gesehen habe?

It literally means, "have a guess who I have just seen" but its actual meaning is more like, "you have no idea who I have just seen" (implying that the other person doesn't know who you have just seen).

Hope that helps. But please note that I am only a native speaker and not an expert in German language ;)