Definitions of back up:

• verb:   make a copy of (a computer file) especially for storage in another place as a security cop

  Example: "You'd better back up these files!"

• verb:   move backwards from a certain position
• verb:   establish as valid or genuine

  Example: "Can you back up your claims?"

• verb:   give moral or psychological support, aid, or courage to
• verb:   become or cause to become obstructed