Excogitative refers to the process of thinking and contemplating deeply about something. Its antonyms, therefore, would be the words that describe a lack of thought or consideration. Some possible antonyms for excogitative include shallow, superficial, thoughtless, impulsive, hasty, and rash. These words describe a lack of careful consideration and suggest a tendency to act without thinking through the consequences. Other possible antonyms might include words like careless, reckless, ill-advised, or ill-considered. All of these words suggest a lack of reflection and deliberation, making them appropriate antonyms for the word "excogitative.