“I’m p-paralyzed with happiness.” She laughed again, as if she said something very witty
There are several layers to this little quip.
First of all, the outermost layer, the literal meaning of what’s going on, is thus: moments prior, she couldn’t decide whether or not to stand up and greet Nick. This indecisiveness is unbecoming of someone so social and graceful as her, and by stuttering slightly (“p-paralyzed”) she is conveying that her momentary lapse in grace is, in fact, a form of paralysis.
You may have noticed that this joke is, in fact, not funny at all. This illustrates that despite her complete lack of wit in this particular instance, the fact that she is very high on the social ladder means that she has every right to laugh at her own terrible jokes and everyone is expected to at least chuckle at it.
http://www.youtube.com/watch?v=EffPnse4WQs The second layer is the poetic irony in this sentence. Earlier in this passage Daisy is portrayed as weightless and free-flowing, but is now claiming to be be absolutely p-p-paralyzed. This weightlessness can be seen as an analogy for social mobility, and the fact that these upper class gentry can go anywhere and do anything they please. However, the irony goes deeper than that. [SPOILERS, YO] Despite the fact that she should be socially mobile and able to do anything that she wants, in the end she stays with Tom Buchanan, a man whom she hates and who was cheating on her, rather than go with Gatsby whom Daisy would almost certainly be more happy with. She is, in essence, truly paralyzed.
To help improve the meaning of these lyrics, visit “The Great Gatsby (Chapter I)” by F. Scott Fitzgerald and leave a comment on the lyrics box