I'm really going to nail his ass now. (2025)

Source: Die Hard (1988), an American movie.

Los Angeles. A group of terrorists takes control of the American headquarters of Nakatomi Corporation. They are holding everyone in the building hostage. Only John McClane, a New York policeman, managed to elude the terrorist's hands. He's hiding somewhere in the building and attacks them whenever he can. But what's more important is that he's stolen the plastic explosives they sorely need.
The LAPD has surrounded the Nakatomi Building with a large SWAT force led by Deputy Chief Robinson. After the first unsuccessful attempt to storm the building, they choose to wait for backup.
All three parties (the terrorist, John, and the police) use CB radio to talk to each other.
The leader of the terrorist, Gruber, uses the radio to contact John, and demands that John return the explosives, or else he will shoot one of the hostages. John tries to reason with Gruber, but in the end he refuses to give up the explosives, and so Gruber kills the hostage. The police force heard everything on radio.

Deputy Chief Robinson to Powell (the policeman who's been in constant touch with John from the beginning): Listen, you tell this partner of yours to stay the hell out of this from now on, because if he doesn't, I'm going to nail him. I'm really going to nail his ass now. Believe me.

Could you please tell me if to nail simply means to arrest in the given context? And what is the "now" for? What does it add to the meaning of the statement here?

Unfortunately, "to nail" has several meanings and is further confused by the ability of "to nail" to form phrasal verbs. The context informs us which meaning "to nail" has.

In this case, to nail (informal) -> (i) "to take decisive action against someone/something."
his ass/arse = him.
Now -> from this moment onwards - with the emphatic implication that whatever has happened has caused the speaker to move from a position of passive disapproval to active retaliation.

Thank you, PaulQ. The meaning of this "now" is clear to me now.

PaulQ said:

In this case, to nail (informal) -> (i) "to take decisive action against someone/something."

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I felt that there was something that didn't add up... Just to be sure, am I correct to think that none of these definitions from WR Random House Unabridged Dictionary of AE perfectly fits here?

11. Informal.
11a. to secure by prompt action;
catch or seize: The police nailed him with the goods.
11b. to catch (a person) in some difficulty, lie, etc.
11c. to detect and expose (a lie, scandal, etc.).

12. Slang. to hit (a person):
He nailed him on the chin with an uppercut in the first round.

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It is similar (i.e. has resemblances) to 11a.

I don't think you will find a perfect synonym or definition that fits all contexts, or that an exhaustive list exists (yet another meaning has occurred to me as I write.). There are a few English verbs in this category - they are called pro-verbs - and you should apply 1. common sense 2. a general principle of what the word implies in its broadest meaning.