"Trick" here means the act of prostitution.
A common phrase used for when a prostitute performs a service for a customer is "turn a trick" or "turning tricks."
This example shortens the phrase, and then sort of turns it into a verb. "Verbing nouns and Nouning verbs" is common in english.
From original question
I thought only " what does@@mean?"was grammatically correct but I heard a lot of Americans saying "what does it mean by @@?"
If you saw something confusing on a street sign, you might point and say "what does it mean by that".
Here, "it" would refer to the sign. You are treating the sign as if it were a person who had said something. So it's very similar to sentences like "What did he mean when he said that?"
This isn't particularly common. If you are reading and trying to understand a book that has a lot of confusing sentences and it's like the book itself starts to take on an evil identity...it might be more common. :-)
As for "What does it mean that (...)", there are rare cases you might hear it as a kind of question to provoke thought, where you're not expecting an answer (e.g. a "rhetorical question".)
- "What does it mean that one fourth of humanity lives without electricity?"
But if you're really asking about what something means, "What does (...) mean?" or "What's the meaning of (...)?" would be what you'd want to use.
From updated question
I thought only "what does _____ mean?" was grammatically correct but I heard a lot of Americans saying "what do you mean by _____?"
Are both of them correct? Are there any differences between those two?
If you ask "what do you mean by ____" then you are suggesting the person has said something you want them to explain. But "what does ____ mean" could be asked of someone whether they've said anything or not. You're just asking a question about the general meaning of something.
In usage, "What do you mean by that?!!?" can be a strong negative reaction...usually when someone has said something that the person feels was meant to be insulting even if it wasn't "obviously" so. So it's not really asking for clarification, because the speaker has already assumed it was meant in a bad way.
But it can also just be a polite phrase asking what someone meant.
Best Answer
King John evidently does not want Robin Hood to speak. Figuring the crowd wants to hear what Robin has to say, and since Robin is eager to speak, the King figures he may as well let him speak.
The expression "if you must" indicates a bit of impatience on the part of the one who says it. This person thinks there are better things to do than what you want to do, but he'll let you do your thing--again, only if you must. Notice, however, that in the case of Robin, the "must-ness" is on Robin's part, not the King's. If the King were to say, "You must not speak," then Robin would not have spoken. The King's words, in that case, are an order. Must-ness, as answerer Jasper Loy, above, suggests, indicates something that has to be done, in the opinion of the one who wants to do it.
Now if the person who wants to do or to say something thinks it can wait, he might say,
in which case, whatever needed to be said or done wasn't that important.
The word must denotes something urgent, but what you consider urgent and what another person (such as the King) thinks is urgent are not necessarily the same. If the person in charge thinks it's not urgent, he may still give his subordinate permission to speak, but only because he has the authority to grant permission.
In conclusion, perhaps an example might help. Keep in mind, the person who says "If you must" is a little impatient, and the person to whom it is said thinks what he has to say or do is urgent.