The phrase 'back up' is commonly used in a range of settings to describe the act of providing support or assistance. There are many synonyms for 'back up' that can be used depending on the context, including 'assist', 'aid', 'help', 'support', 'back', 'reinforce', 'validate', 'confirm', 'substantiate', 'corroborate', 'certify', and 'vouch for'. In workplace settings, 'cover', 'substitute' and 'relieve' are also common synonyms for this phrase, while 'back off' or 'withdraw' can be used in more confrontational scenarios. Whatever the context, there are a wide range of phrases that can be used as synonyms for 'back up' depending on the situation.