I. intransitive verb
1. of water checked by an obstruction : to rise and flow backward or overflow adjacent areas
clogged pipes caused drain water to back up into the house
a dammed stream that backs up and floods a meadow
2. of nonliquid objects : to accumulate through lack of an outlet in undesirable or unmanageable excess or in a congested state
supplies are ample, if not already backing up in the hands of the producers — Biddle Survey
cars back up for blocks on either side of the Main Street traffic light — Louise Levitas
3. cricket
a. of a batsman : to move forward of one's crease in readiness to run
b. of a nonstriker : to so move as the ball is delivered
transitive verb
: to hold back (as a river) usually causing an accumulation
this dam … backs up billions of gallons of water to prevent floods — N.M.Clark
II. transitive verb
: to make a backup of (a computer file or data) to protect against accidental loss ; also : to make backups of all the files on (a hard disk)