The antonyms for the word "most closed up" can be interpreted in various ways depending on the context. In terms of emotions, the antonyms for being closed up could be described as open, expressive, talkative, lively or communicative. For an enclosed space like a room or door, the antonyms may include open, unobstructed, accessible, exposed or free. In contrast, for an object or container, the antonyms may include empty, vacant, hollow or revealed. It's important to consider the context when trying to identify antonyms, as they may differ depending on whether you're describing emotions, spaces or objects.