• accumulate; cause to accumulate; provide support; substantiate; move backwards; move a vehicle in the reverse direction; support or confirm someone's testimony; make a copy of an original to have in case the original gets damaged or lost (esp. about computer files)
• backing, support, aid
• back-up is extra help from people or machines which you need in order to be able to achieve something.
• if you have something such as a second set of plans as back-up, you have arranged for them to be available for use in case the first one does not work.
back up a car
• move a vehicle in the reverse direction
back up light
• white light at the back of a car that lights when the car is backing up or is in reverse, reversing light
set one's back up
• increase one's opposition, strengthen one's objection
to get one's back up
• be steadfast about, be stubborn about