Zip by is an expression that typically refers to something moving quickly or passing by rapidly. The word "zip" implies a sudden movement, which is further emphasized by the word "by". Some synonyms that could be used for this phrase include "whizzing past", "flying by", "speeding along" or "racing through". Each of these suggests something or someone moving rapidly through a particular area, such as a car on a highway or a cyclist on a race track. No matter which word you use, the underlying message remains the same - something is moving swiftly and without delay.