Hi, Never heard of this idiom before, but I am pretty sure it means 'don't give it all away for nothing'. Supposedly, I guess, because by showing Chandler her cleavage, Phoebe would lose all of her leverage while getting no information in return. - DJB -
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I think 'Don't sell the farm' means don't give away cheaply what people should be charged for at some cost. ie don't give away photos of yourself for nothing. Don't give your body away for nothing. Get married first. In the Friends version it might mean don't get pregnant.