an expression used in response to someone talking complete rubbish.
It was originally used in response to clueless ramblings of the male sex but became a universal phrase with the ignorant shouting of a mad-woman.
ie "you want this grape about as much as i want to continue listening to what you have to say!"
other person: "you are useless, you have no idea about anything, you are inconsiderate...blar blar blar...."
you: "have a grape!!!"
(expected result: complete bewilderment, rambling to be paused...blissful silence!!, and possibly if ur lucky some laughter!!!!)