v. 1. To move backwards. The train was backing up. 2. To help or be ready to help; stay behind to help; agree with and speak in support of. Jim has joined the Boy Scouts and his father is backing him up. The principal backs up the faculty. Jim told us what had happened and Bob backed him up.
Compare: BACK OF 3 , STAND BY 4 . 3. To move behind (another fielder) in order to catch the ball if he misses it. The shortstop backed up the second baseman on the throw.