If you want to know what Puritans thought, you should consult the theologian that they considered supreme: John Calvin.

In Calvin's Commentary on First Corinthians, he says, "None can deny that Dionysius (whoever he may have been) has many shrewd and subtle disquisitions in his celestial hierarchy, but on looking at them more closely, everyone must see that they are merely IDLE TALK. The duty of a theologian, however, is not to tickle the ear, but confirm the conscience, by teaching what is true, certain, and useful."

I would say that untrue, uncertain, and not useful are a good working definition for idle talk. If you are passing along rumors, those are idle talk. If you are speculating, it could be idle talk. If you bring up irrelevant points, that is idle talk. If you are flattering someone with fanciful speech, it's definitely idle talk. If you are telling someone what they want to hear but you know it's not true, that would be idle talk.