One of the best dialogues I've heard. Here it is:
Tony: Hey, it's your money. I mean, I'm just holding it for you, like a bank. Except better than a bank, 'cause you know banks always get knocked off. No one knocks off old Tony.
But Tony got knocked off by DA officer and he shared Leon's location, didn't he? Or did he say that to mean that nobody stole any money from him and Leon's money is safe with him. Well, the money was safe but Leon's safety wasn't so secure.
Or did he say that to mean that nobody stole any money from him and Leon's money is safe with him.
This is exactly what Tony is saying.
It's implied that no-one would dare try an rob a mafioso like Tony whereas the a bank could get robbed anytime.
It's also a method for Tony to maintain control over Léon by withholding access to the money to ensure that he continues working.
Tony also passes the money (in allowance form) to Mathilda in accordance with Léon's wishes after he is killed...and again, Tony's "control" is maintained, this time over Mathilda...potentially a new "cleaner" for him.
Knocked off means to hit, strike or thump. In this sense, it is slang:
Slang. to commit a robbery at; steal from: The gang knocked off a gas station.
Of course Tony is exaggerating, because not all banks get robbed, and no one is untouchable. Crooked cops are more dangerous than any Mafioso as we see in the film.
The second meaning is to kill. Someone knocking off Tony could mean someone steals from him, or attacks and kills him.
