i first heard it on SNL...it is from a judge judy skit...cheri o'teri is one of the greatest snl cast members ever
"DON'T PEE ON MY LEG AND TELL ME IT'S RAINING!"
also, this is the title of one of judge judy's real books. haha.
i just think that this is such a great quote, and it should be integrated into society and be just as popular as twitter.