Bridge Too Far Meaning

(idiomatic) A step or action that is too ambitious; an act of overreaching.

Example: 2006, Adair Broughton, Crossing the Bridge of Infinity (page 40)
  Cloning was an interesting concept. Cloning body parts was again just man's search for immortality riding on the back of naive altruistic medicinal benevolence. But cloning a person was a bridge too far.