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Harry had given a cry of pain; his scar had burned again as something flashed across his mind like a bright light on water. He saw a large shadow and felt a fury that was not his own pound through his body, violent and brief as an electric shock.

_{Harry Potter and the Deathly Hallows}

What does 'pound' mean in this context? I've looked it up in free dictionary. But I don't know which definition fits.

The term "pound" in this instance means to pulsate or throb. The sentence could have been written:

"He felt ... fury ... throb through his body".

or

"He felt ... fury ... pulse through his body".

We often refer to blood "pounding" in a person's veins when a person is angry or fearful, because of the faster and harder heartbeat that is created by anger or fear. The sentence above has an implied or sub-textual reference to a pounding heart or blood pounding through the veins of the person who is experiencing fury.

See definition of verb "to pound".

You parsed it in error. It's not

(not) felt 
(not) a fury 
(not) that is not his own pound
(not) through his body

The noun is not "pound" being modified by "fury".

It is "fury" being modified by "pound".

felt 
 a fury 
 that is not his own
 pound
 through his body

"Pound" is what the fury is doing. It's an action. Note that two phrases are modifying "fury": "that is not his own" and "pound".

If we simplify the sentence as much as possible by removing modifying phrases (except pound), we get

He saw a shadow and felt a fury pound.

Or simpler,

He saw a shadow and felt a fury.

"pound" is a verb form of "to pound" here. It is the same construct as "I felt a bee sting me". The core part means "He felt a fury pound through his body, a fury that was not his own". I'd use "rush" instead of "pound" here maybe, because furies tend not to move in one's body in much of a pounding manner.

The answer by Harper provides an excellent technical analysis. For simplicity, the problem of comprehension can also be resolved by the minimal use of punctuation for clarification: "He saw a large shadow and felt a fury that was not his own, pound through his body".

I have done work translating German technical documents into English. As a result, I would not want to translate an English document into German.