This is a question about American English usage of the word "already". As a UK resident I don't completely understand when I hear Americans give commands like "Stop it already!" In the UK the word already is not normally used in the imperative mood and the sentence I've just quoted would leave an English person thinking "If you're saying I've already stopped it why are you asking me to stop it again?"
The implication of an imperative with "already" is that the proper time for carrying out the command has passed, and the person being spoken to is remiss in waiting to be told:
Bob: Oh, I really should be going. My meeting starts in a minute.
Mary: So leave already!
Mary's point is that Bob should have left some time ago, and she's annoyed by his delay.
Be careful when using this construction. Saying it to your boss, for example, if he is an American and at all ill-natured, would be a CLM, a Career-Limiting Move.
Not only is already used in this way informal, it can be downright peremptory and even aggressive, but it is always imperative. It indicates that the speaker is out of patience and wants to end this part of the conversation and proceed to the next stage (or to exit it altogether).
Stop being such a jerk already!
Shut up already! We've gone over the details, now let's just do this thing.
Are you going to let him push you around like that? Grow a pair already! Fight back!
Now, it doesn't have to be an angry or negative statement. It can be spoken among friends
You want me to introduce you to that chick? So buy me a beer already.
Dude! I haven't seen you in, like, months. Sit your ass down already and tell me what's shakin' with you.
Yeah, it was a great party. But enough talk — unzip my dress already!
I like to read too, but right now I want you to put down that book and give me a kiss already.