You just don't get it, do you?
Get what? Get outta here.
ADDED: "Get" in the you just don't get it sense first appeared in 1892, according to the Oxford English Dictionary (which I'm sorry I can't link to).
[1892 ‘M. Twain’ Amer. Claimant xiii. 101, I don't know that I quite get the bearings of your position.]"Get" in the get outta here sense is older:
1907 M. C. Harris Tents of Wickedness i. iii. 33 ‘I don't get her,’ she murmured, as if Leonora was a telephone number....
1918 P. G. Wodehouse Piccadilly Jim xi. 114, I get you not, friend. Supply a few footnotes....
1956 I. Bromige Enchanted Garden II. ii. 93 Fiona broke into peals of laughter and became quite helpless for a few moments. ‘Don't get it,’ said Julian.
1711 R. Molesworth tr. F. Hotman Franco-Gallia (1721) 136 You have nothing to do here (said she): get out!
1841 Dickens Old Curiosity Shop i. x. 143 Kit only replied by bashfully bidding his mother ‘get out’.