That is a very good question.. "Pull a fast one" means to do something sneaky, to deceive or do something outside the expected rules of honest behavior.I suppose the image is a magician doing a trick so fast that you don't see his hands move and therefore realise you've been tricked.

pull a fast one . to succeed in an act of deception. e.g She was pulling a fast one when she said she had a headache and went home. Don't try to pull a fast one with me! I know what you're doing. to successfully deceive someone