The word "unzip" denotes the act of opening or undoing a zipper. As such, its antonyms would be words that denote closing or doing up a zipper. Some of the antonyms for "unzip" include "zip up," "fasten," "close," "clasp," "button up," and "seal." These words help express the opposite meaning of "unzipping." For instance, when getting ready for a formal occasion, you might say, "I need to zip up my dress," instead of "I need to unzip my dress." Similarly, when packing a suitcase, you might say, "I need to fasten my luggage" instead of "I need to unzip my luggage.