The phrase "doing top of your head" refers to thinking or responding without giving much thought or consideration. Some synonyms for this phrase include reacting impulsively, speaking off the cuff, making snap judgments, or giving quick, unconsidered responses. It can also be described as making unguarded or hasty comments, acting thoughtlessly, or jumping to conclusions. These phrases imply a lack of forethought or reflection and suggest that the person speaking or acting may not have considered all the facts or information before responding. Expressions like these can be helpful in highlighting the need to be more mindful and thoughtful when making decisions or giving responses.