The term "on nail" generally refers to something that is secure and stable. Antonyms for this term would be words that describe things that are unstable or insecure. Words like wobbly, rickety, unsteady, shaky, precarious, and uncertain come to mind. These words can describe physical structures like a chair or a building that is not durable, but they can also describe abstract concepts like someone's job security or financial stability. Other antonyms for "on nail" might include unpredictable, unreliable, or unsafe. These words can all be used to describe anything that is not firmly established, either physically or intangibly.