Batten down is a phrasal verb which means to prepare for a storm or dangerous conditions by securing loose objects or closing up a building. The antonyms for batten down are "open up" or "loosen." In other words, when you open up something, you are making it more vulnerable to damage by wind, rain, or other environmental elements. On the other hand, when you batten down, you are making something secure and resistant to the elements. Loosen means to make something less tight or compact, while batten down means to make something more secure and tight. Overall, understanding antonyms for batten down can help one use the right words in appropriate circumstances.