back somebody up

Từ điển Oxford Advanced Learner 8th

ˌback sb/sthˈup derived
1. to support sb/sth; to say that what sb says, etc. is true
• I'll back you up if they don't believe you.
• The writer doesn't back up his opinions with examples.
2. to provide support for sb/sth
•two doctors backed up by a team of nurses
• The rebels backed up their demands with threats.
related noun ↑backup
Main entry: ↑back_derived