Antic disposition
I'm Hamlet. I'm a smart aleck with a penchant for delivering long speeches about the meaning of life. I'm pretty depressed since my dad died and my mom married my uncle. And you know what I think?
And therefore as a stranger give it welcome.
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy. But come;
Here, as before, never, so help you mercy,
How strange or odd soe'er I bear myself,
As I perchance hereafter shall think meet
To put an antic disposition on,
That you, at such times seeing me, never shall,
With arms encumber'd thus, or this headshake,
Or by pronouncing of some doubtful phrase,
As 'Well, well, we know,' or 'We could, an if we would,'
Or 'If we list to speak,' or 'There be, an if they might,'
Or such ambiguous giving out, to note
That you know aught of me: this not to do,
So grace and mercy at your most need help you, Swear. (1.5.165-180)
Who Said It and Where
Welcome to Elsinore, Denmark, land of a recently deceased King who likes to chill out in ghost form at night on the castle battlements. He has reason to be upset, though, since the new King Claudius, husband of Queen Gertrude, happens to be his own brother. (Must make family holidays complicated.) To be fair, Claudius has problems, too: Norway's Prince Fortinbras has war on the brain, and his new stepson, Hamlet, is being a bit of a PITA about things, mostly because his mother (Gertrude) waited a grand total of about two seconds to get married after his father died.
Hamlet's bud Horatio tells Hamlet about the ghost and arranges a meeting. The ghost (who says he is indeed the ghost of Hamlet's father) gives young Hammie the skinny on what really went down in Denmark: Claudius killed him, and then married is wife. Needless to say, he's more than PO'd. He wants revenge, and sooner rather than later, since he's doomed to burn in eternal flames until he gets it. And you know who's gonna get it for him? Hamlet, of course.
Let's all take a moment to underline this or highlight it or whatever you do. Hamlet promises to avenge his father. He does, no fingers crossed behind his back or anything. Got it? Good. That's the moment the rest of the play hinges on, and it sets up the "antic disposition" business, which is the reason you're here in the first place.
Hamlet comes back from this revealing rendezvous and swears his friends to secrecy. He also warns them that he might start behaving strangely, but it's cool: he's just acting on instructions given to him by the ghost of his dead father. Specifically, he says he's going to put on an "antic disposition," which means he's going to pretend to be crazy, like an "antic" (which is the role of a theater clown or a grotesque).