What does "plunged recklessly on" mean?

Question

'Professor Snape's very interested in the Dark Arts,' he blurted out.

'Really?' said Lupin, looking only mildly interested as he took another gulp of potion. 'Some people reckon —'

Harry hesitated, then plunged recklessly on, 'some people reckon he'd do anything to get the Defence Against the Dark Arts job.'

After researches in dictionaries, I found this sense of 'plunge' might fit for the context:

6. (intr) informal to speculate or gamble recklessly, for high stakes, etc

But I haven't found a reference anywhere about the phrase "plunge on". Can we just remove 'on' from the context: Harry hesitated, then plunged recklessly, ...

What does it mean exactly in this context?

Answer 1

"to plunge" is to dive headfirst into a body of water or to move with some momentum forward, into a thicket, say, or into a crowd. It can be used figuratively of a topic of conversation in place of the body of water or the thicket or crowd.

He plunged headlong into what was a rather delicate conversation.

There's no stopping you from entering the water when you plunge, or from entering the underbrush once you have the momentum of the plunge. Under certain circumstances, you may be acting with abandon or imprudently when plunging, hence "recklessly". There is the element of the unknown. You don't know what is is under the water or in the dense brush.

In this Harry Potter example, Harry is speaking, and speech being something which progresses as more and more is said, we find on, which denotes forward progress. The preposition is not absolutely needed, but it does convey the idea that Harry continued to speak, not merely that he had made the decision and had begun to speak, which is all that plunged would convey.